relationship with the formation of Gondwana
By RoelfJ. Mulder
University of Utrecht
Faculty of Geoscience
September 2006
1. Abstract 2 2. Introduction: 3 3. Gondwana 3 4. Geology of Madagascar 5 4.1 Introduction 5 4.2 Antananarivo Block 5 4.3 Tsaratanana Sheet 8 4.4 Neoproterozoic Sedimentary Belts 9
4.5 Antongil Block 9 4.6 Bemarivo Belt 9 4.7 Deformation History 11
5. Fieldwork 13 5.1 Introduction 13 5.2 Geology and Morphology 13
5.3 Lithology 14 5.4 Petrology 16 5.5 Structural Features 16
5.6 Unstraining of D3 19 6. Lattice-Preferred Orientation of Quartz 21
6.1 Introduction 21 6.2 LPO patterns, some theoretical background 21
6.3 Methods 22 6.4 Microstructural Analysis 22
6.5 C-axis Measurements 25
7. Monazite Dating 25 7.1 Monazite Occurrence 25
7.2 U - Th - Pb dating with monazite 25
7.3 Electron Microprobe 26 7.4 Methods - Electron Microprobe Analysis (EMPA) 26
7.5 Age Calculations 27
7.6 Results 28 8. Discussion and Conclusions 28
8.1 Imphcations of fhe Petrology of the Study Area 28 8.2 Implications of the structural history of the study area 31
8.4 Implication from LPO in quartz 34 8.3 Implication from Geochronology 34
8.5 Conclusions 37 Acknowledgments: 38
MULDER 2
1. Abstract
2. Introduction:
Lying off tlie coast of Mozambique, Madagascar is the fourth largest island in the world and has a very rich geological history. During the Neoproterozoic to Cambrian times a supercontinent Gondwana was formed (Poweh et al. 19931 Shacldeton 1996; Collins & Windley, 2002; Kroner & Cordani, 2003), which was the result of the final collision between Africa, India, Antarctica, and Madagascar (Fig. 1). During this process the Mozambique ocean closed and cratons, arcs and ocean floor material were all amalgamated into one mountain belt, namely the East African Orogeny (EA0)(Stern, 1994), which reaches from Arabian-Nubian Shield in the north to Madagascar and even Antarctica in the south (Kuslcy et al., 2003). Being trapped between India and Africa and in the middle of the EAO (Fig. 1), Madagascar is consequently one of the most important locations to find evidence for the processes playing a part in the formation and the break-up of Gondwana (de Wit et al., 2001; Collins & Windley, 2002; Collins et al., 2003).
The western coast hne and the minority of the geology of Madagascar consists of sedimentary basins capturing sedimentation from the Carboniferous to present (de Wit et al., 2001). However the bigger part of the island is made up of tectonic units consisting of Precambrian rocks which all have experienced the EAO event (de Wit et al., 2001). The major tectonic units are the Antongil Block, the Antananarivo Block, the Bemarivo Belt and Tsaratanana Sheet with the south consisting of Neoproterozoic supracrustal belts and several shear zones (Collins et al., 2000; Colhns & Windley, 2002; de Wit, 2003).
The study area is located south of the 'Massif de Tsaratanana', within the tectonic contact zone between the Bemarivo Belt, the Antongil- and the Antananarivo Blocte as indicated in fig. 2 and 4 (Colhns, 2006). This tectonic boundary, separating the Neoproterozoic Bemarivo Belt with the Archean rocks of the Antananarivo- and the Antongil Block, has thus far been identified as a top-to-the south thrust contact (Colhns & Windley, 2002), which according to cooling ages occurred in between 520 and 510 Ma (Buchwaldt et al., 2003). Due to the fact that the emplacement of the Bemarivo Belt is beheved to have taken place during the formation of Gondwana, the Idnematics that its southern border has experienced might give additional constraints for the reconstruction of the collision between East and West Gondwana and the consequent formation of Gondwana. The lack of research concerning this thrust, therefore makes this area and subject very interesting. The research consequently focuses on applying structural analysis i n the field in combination with geochronology by monazite dating carried out at Utrecht University. The data collected is used in order to make a reconstruction of the deformation history concerning the Bemarivo Belt and establish the timing and the Idnematics of the tectonic contact dividing the Bemarivo Belt, with the Archean Antananarivo- and Antongil Blocks.
3. Gondwana
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Fig. 1 - Gondwana reconstruction of the Neoproterozoic arrangement of the continental blocks and cratons participating (Colhns et al., 2003).
countries and other Gondwana players. Furtlier research by, among others. Kroner (1984) who included orogenic events between 950-450 Ma to the term Pan-African, lead to the concept of a supercontinent that formed during this period, which was named Gondwana. With the help of similar geochronology, paleomagnetics and structural- and metamorphic data found on the different continents where Cambrian and Neoproterozoic rocks were still accessible, the reconstructions were consequently made for the evolution of Gondwana (e.g. Stern, 1994). Despite the extensive research concerning this subject, a lot of uncertainties are still present related to the exact sequence of events and the precise role and the arrangement of the different cratons and continents involved.
p/a Afdeling Geotechnoiogie Postbus 5028
2600 GA Delft Van: Roelf Mulder
Flat 7, 6 Burston Road Putney
London SW15 6AR
Onderwerp: Verslag aangaande het 'Madagaskar' project
Geachte M.J. Schillemans- van Tuyl,
Zoals afgesproken stuur ik hierbij mijn afstudeer onderzoeks verslag, de laatse eis waaraan ik nog moest voldoen aangaande de beurs van €1000 die ik heb mogen ontvangen van het Molengraaff Fonds voor het Madagaskar veldwerk gedurende september en oktober, 2005.
Bij deze wil ik ook het Molegraaff Fonds en haar verantwoordelijken bedanken voor de fmanciële steun wat betreft dit project. Zonder deze steun kon ik mijn idee nooit
bewerkstelligen en zou het noordelijke stukje Madagaskar nog lang onbekend blijven. Verder was dit project een zeer leerzame en rijke ervaring die ik dankzij het Molengraaff Fonds mee mag nemen in mijn verdere carrière.
Succes met het verdere veloop van het Molengraaff Fonds. Hopelijk voldoet mijn verslag aan de eisen. Zo niet dan kunt u contact met mij opnemen op het bovenstaande adres.
Madagascar to Antarctica (Kuslcy et al., 2003). The second step in the formation of Gondwana was the collision of East Antarctica and Australia with the still poorly defined East Gondwana. This lead to the Kuunga Orogeny dated at 570-530 Ma (Meert, 2003). The major events are relatively well established, which can not be said about the smaller events in between the major orogenies. According to the two-step formation of Gondwana (Meert, 2003), East Gondwana could hardly be defined by one name, considering the fact that Australia and Antarctica were not yet attached. This is supported by earlier research claiming that in between the formation of the East African and the Kuunga Orogeny, East Gondwana was made up of different parts (Meert et al., 1995; Meert & Van der Voo, 1997).
Scattered paleomagnetic poles were namely found in the rocks of EAO-age of the concerning terrains. This is in masked contrast to the poles corresponding to rocl<s younger than 550 Ma (after the Kuunga Orogeny), which were more concentrated and therefore imply a unified East Gondwana (Meert et a l , 1995)- This concept was further supported by the different timing found for the granulite-grade metamorphism that corresponds to the closure of the Mozambique Ocean. The age of this peak metamorphism becomes younger further from the EAO zone suggesting that individual terrains collided with each other at different times during the formation of the EAO (Kroner et al., 2001; Colhns & Pisarevslcy, 2005). The oversimplification did not only concern the term 'East Gondwana' but also the group of continental blocks called 'West Gondwana'. The formation of West Gondwana, consisting largely of South America and African blocks, was namely finalized at least around 600 Ma according to Trompette (1997). However, the colhsion of East and West Gondwana occurred before 600 Ma, indicating that the name 'West Gondwana' was also not validated (Meert, 2003).
The badly established arrangement of the continental blocks, participating in the formation of Gondwana, makes research in the area very interesting. Especially when dealing with a craton belonging to Madagascar, which was positioned in the middle of the EAO and therefore contains a lot of evidence in unravelling this geological problem.
4. Geology of Madagascar
4.1 Introduction
Besides the Phanerozoic sedimentary basins along the west coast of Madagascar, which are not of interest to this research, the majority of the geology on this island consists of rocks of Pre-Cambrian to Cambrian age belonging to several tectonic units which are divided by thrust contacts, shear- and mylonite zones. These units include: the Antananarivo Block, the Tsaratanana Sheet, the Antongil Block, the Neoproterozoic metasedimentary belts in the southern part, and the most northern Bemarivo Belt (Fig. 2; Collins, 2000; De Wit, 2001; Collins & Windley, 2002).
4.2 Antananarivo Block
Granulite-MULDER 6
Bemarivo Belt
Tsaratanana Sheet
Antananarivo Block
Antongil Block
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grade metamorphism was subsequently active between 700 and 532 Ma, overlapped by (again) a period of granitoid intrusion between 630 and 561 Ma characterised by loom- to km-scale bodies, a characteristic feature of the Antananarivo Block (Paquette & Nédélec, 1998; Nédélec et al., 2000; Collins & Windley, 2002). The granitoid formation was coeval with a period of extension which is associated with the Betsileo shear zone, now marking the eastern boundary the Itremo Group with the Antananarivo Block (Fig. 3). The transcurrent deformation associated with the so called 'Zone de Virgation' dated at ~56o Ma by the remagnetization of the 'stratoid' granites and the formation of the Angavo Belt in the east (Paquette & Nédélec, 1998; Kroner et al., 2000; Meert et al., 2003). The age of this event is constrained by the youngest granitoid gneiss dated (~55i Ma) and the age of the underformed Carion granite (~538 Ma)(Kröner et al., 2000).
The Itremo Group, which lies geographically south of the Antananarivo Block (Fig. 3), can be seen as an independent tectonic unit due to its different deformation history but due to similar age it is considered as part of the Antananarivo Block. The Itremo Group consists of a basement of amphibolite and gneiss, which can be correlated with the orthogneiss rocl<s of the Antananarivo Block, followed by metasediments and finally overlain by metavolcanics (Collins & Windley, 2002; Collins, 2006). The metasediments consist of quartzites, pelites and dolomitic carbonates, dated at 1855 ± 11 Ma and were therefore probably deposited during Proterozoic times before the formation of Gondwana. Due to the similarities between sequences on the African mainlands, the depositional environment could be a passive margin. However, this cannot be established due to the lack of basin boundaries within the Itremo Group (Cox et al., 1998). The metasediments where deformed into large-scale recumbent folds, separated by mylonitic shear zones. The metamorphic grade increases from sub-greenschist rocl<s in the east to l<yanite-and sillimanite-bearing roclcs in the centre and the west of Itremo (Cox et al., 1998). The metasediments where intruded by syenites and gabbros at 804-779 Ma, which show a subduction-characteristic chemistry similar to the Antananarivo Block but are less deformed (Collins, 2000; Collins & Windley, 2002; Collins, 2006). A second deformation occurred after 789 Ma, leaving open, upright folds, divergent reverse faults and strike-slip faults, which were finally intruded by granitoid bodies (Collins, 2002; Collins, 2006). I t is remarkable that no extensional deformation has been active in the Itremo Group as opposed to most of the rest of Madagascar. An explanation for this fact could be that extension was compensated by sliding along the Betsileo shear zone, i.e., the east boundary of the Itremo Group (Collins, 2000).
4.3 T s a r a t a n a n a Sheet
separated by a mylonitic zone from the underlying Antananarivo Block. Locally this zone shows evidence for a top-to-the east movement, namely a thrusting, but the only constraint on the timing of this event is the magmatism found in most of Madagascar around 780 Ma (Collins, 2000; Collins & Windley, 2002). However, this is again contradicted by the lack of 'stratoid' sills in the Tsaratanana Sheet, which intruded Antananarivo rocte at 630 Ma. This puts the placement of the sheet after 630 Ma (Goncalves et al., 2003).
4.4 Neoproterozoic Sedimentary Belts
The southern part of Madagascar consists of Neoproterozoic sedimentary belts divided by several shear zones, namely the Ampanihy, the Vorokafotra and the Ranotsara shear zone (fig. 3). On the east the Betsileo shear zone marles the boundary of the Antananarivo Block (Collins, 2006). The supracrustals that form belts are indicated in the geological overview and consist of the Vohibory (-850 Ma), the Androyen (before 630 Ma) and the Molo units (620-560 Ma) (fig. 3). The sediments reflect foreland basin, oceanic/arc and volcanic rift environments in the Neoproterozoic (de Wit, 2001; Collins, 2006). This is consistent with the Itremo Group to the north which was deposited earher in passive or Atlantic-type continental shelf environment (de Wit, 2001). Between 650 Ma and 610 Ma these units experienced a compressional event in combination with granulite grade metamorphism, which was followed by orogenic collapse after 590 Ma. The shear zones that are present in this group are probably related to a transpressive regime (associated with e.g. flower structures) dated at 590-500 Ma (Martelat et a l , 1999) which can probably be linked with the 'Zone de Virgation'.
4.5 Antongil Block
Lying structurally below the Bemarivo and the Antananarivo Block (Collins, 2000) ,the Antongil Blocl<s marles most of the East coast of Madagascar. It is a block that consists of a gneiss and granitic core with metasediments along its western and northern boundary,. The ortho- and paragneisses in the core were dated at 3127 Ma and were consequently intruded by granites of -2522-2495 Ma (Tucker et al., 1999)-The rocks are generally low metamorphic, namely of greenschist to lower amphibole grade (Tucker et al., 1999; Collins, 2000) hut seem to have undergone no Proterozoic metamorphism like in the rest of Madagascar (Collins, 2000). A remarkable feature which lies in between the Antongil- and the Antananarivo Block is a zone of graphitic pelites with podiform harzburgites, chromitites, and emerald deposits This zone has been named the 'Betsimisaraka suture zone', because it is believed to be the remnants of the Mozambique ocean, collected during its closure between 630 Ma and 530 Ma (Collins, 2000).
4.6 B e m a r i v o Belt
MULDER 10
I I
p-haneioimc oove?Neoproterozoic r ~ n Dadii i Ml' mc 1 Gfuup
f/.»ndïfklM"''-B;ï';f''Isa .lUin-^r.ï Group
I ^ {(Jrtho'jnf»' s. Q f . u i i f . ' K J ' . MjpMJ f us* »1 ftn ks)
K j S . nib S,ïh/i(tl uia Grt-up
•
Archean
fïH M iJU!*:? afid LaJn A'chodO L 1 i CD 3 1' ro 50
Fig. 4 - Simplified geological map of North Madagascar (Buchwaldt et al., 2003).
east are greenschist to granulite grade gneiss dated at 520-510 Ma, are associated with the accretion of an island-arc terrain during the collision between Bemarivo and Malagasy mainland (Buchwaldt & Tucker, 2001; Buchwaldt et al., 2003). The concerning gneisses i n the west are older and have indicated a subduction related origin according to their whole-rock composition (Buchwaldt et al., 2002). To the north and structurally overlying the Sambirano-Sahantana Group are granodioritic gneisses which crystallized at -750 Ma (Buchwaldt et al., 2003). These orthogneiss rocks are referred to as the Mananbato-Base Group and have been subjected to amphibolite-grade metamorphism dated at 511 ± 5 Ma by Buchwaldt et al. (2003). Meta-volcanosedimentary rocks, emplaced at -715 Ma, comprise the rest of the Bemarivo and are collectively called the Daraina Group (Collins, 2000).
(Collins, 2006) and various isoclinal folds in the south-east (Buchwaldt et al., 2003). The tectonic significance of these structures is uncertain, however. Amphibolite to granulite-grade metamorphism and amphibolite-grade metamorphism at 520 -510 Ma observed, respectively, in the Sambirano-Sahantana metapelites and the Mananbato-Base Group were subsequently linked to a presumably top-to-the south thrusting placing the Bemarivo Belt structurally on the Betsimisaraka suture zone, Antananarivo-, and the Antongil Block (Collins et al., 2001; Buchwaldt et al., 2003). This is evident due to the much younger ages found in the Bemarivo Belt in comparison with the Antananarivo- and the Antongil Block and due to the rapid cooling rates found, which could only be accomplished by fast tectonic exhumation (Buchwaldt et al., 2003). According to Collins (2006) this granulite-grade metamorphic event is related to the final closure of the Mozambique Ocean.
4.7 Deformation H i s t o r y
The deformational history relevant to this project is linked to the Gondwana formation. Bringing together tectonic research from different localities in Madagascar, a couple of deformation phases can be recognised to have influenced most, if not all, of Madagascar. An overview of the different events in the different blocks is shown in figure 5. The first significant tectonic event active in significant parts of Madagascar occurred around 800-700 Ma and concerned extensive magmatic activity (Meert, 2003). These gabbroic and granitoid intrusions can be found in the Tsaratanana-, Itremo Sheet and the Antananarivo Block (Collins, 2000; Collins et al., 2001). The origin of these intrusions are still questionable. The Itremo Sheet and the Antananarivo Block show a subduction related chemistry which could be linked to arc magmatism due to the subduction of the Mozambique ocean (Kroner et al., 2000; Collins et al., 2001). However, magmatic underplating following the generation of plumes and magmatism related to the break-up of Rodinia have also been suggested as origins (Kroner et al., 2000).
The chronologically next step in the Gondwana evolution is the EAO when Madagascar cohided with the mainland of Africa. This event is observed as a regional granulite-grade metamorphism dated at 650-630 Ma (Meert, 2003). However, it is remarkable that the timing of this metamorphic event becomes younger to the east of Madagascar, which again supports a divided East Gondwana during the collision (Collins & Pisarevsley, 2005). The mountain building event was immediately followed by, presumably, orogenic collapse leaving extensional structures and consequent granite sills at 630-561 Ma, which are loiown as the 'stratoid'granites and characteristic for the Antananarivo Block (Paquette & Nédélec, 1998; Nédélec et al. 2000). This extensional regime is however not observed to the south and west of the Betsileo shear zone, along which the post-collisional extensional movement was probably localised (Colhns, 2000). In the NeoProterozoic supracrustals compressional events, also correlated to the closure of the Mozambique Ocean (de Wit et al., 2001), took place in a period of 647-609 Ma. However, a period of 'static annealing' until 530 Ma was suggested to be linked to the orogenic collapse (de Wit et al., 2001). Coeval to the orogenic collapse (between 630 and -530 Ma), the east of the Antananarivo Block experienced contraction due to the collision with the Antongil Block, concentrated along the Betsimisaraka suture zone (Collins, 2006).
T i m e D e f o r m a t i o n liistory of (Neo-) Proterozoic Events
Bemarivo A nto nail Tsaratanana Antananarivo NeoProt. Supracrustals
500 510 520 530 540 550 560 570 580 590 600 Top-to-south thrust ® 0 ToB-to-south thrust ® 0 Transpression regime 4' O ® 500 510 520 530 540 550 560 570 580 590 600 Top-to-the-east O Transcurrent •f O ® Ivl ov ement O © Along Post-collisional O ®
I^N-S shearzones extension O ® 1^
Kuunga Orogeny 500 510 520 530 540 550 560 570 580 590 600 thrusting (formation of Betsimaral<a suture)' O O O O ® 4^ O 4- O ® Movement Orogenic O ® along 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 i^Top-to-the-east thrust
Granitoid Collapse O ® Betsileo intrusion 1^ O 0 Shearzone 4^ Compressional O ® 4-610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 ^^Oranitoid intrustiort O 0 4^ O event O O 4^ O EAO 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780 790 800 •f-Granite magmatism si/ ^ O Gabbro intrustion O O 4' O M agmatic event with subduction chemistry
Figure 5 - This diagram shows the (Neo-)Proterozoic deformation history of the different hlocks of Madagascar according to several authors (e.g. Meert, 2003; de Wit, et al,
with high- grade metamorphism in close association with transcurrent movement along N-S shear zones and the formation of the 'Zone de Virgation' in central and south Madagascar (Paquette & Nédélec, 1998; Kroner et al., 1999; Lardeaux et al., 1999; Martelat et al., 1999; Meert, 2003), and the top-to-the south thrusting of the Bemarivo Belt over the Antananarivo and the Antongil Block (Tucker et al., 1999; Collins, 2000). The transpressional regime starting at -530 Ma is observed by the rotated structures in the east and south of Madagascar (Paquette & Nédéled, 1998; Kroner et al., 2000; Meert, 2003) and by positive-flower structures and consequent exhumation prevailing until -500 Ma in the south (Lardeaux et al., 1999; Martelat et a l , 1999). Post orogenic magmatism and cooling from 550 Ma tih 490 Ma marks the last Pan-African related activities (Tucker et al., 1999; Kroner et al, 2000; Meert, 2003).
5. Fieldwork
5.1 Introduction
Using the topographic maps (1: 100 000 of area T35 & U35) of the National Geographical and Hydrological Institute FTM (Foiben-TaosarintaninT Madagasikara) in combination with the 1: 500 000 geological map of Besairie (1971)» the best location for the fieldwork was chosen. The area of research was chosen to be in the vicinity of the tectonic contact between the Bemarivo Belt and the Antananarivo Block and additionally would have to be near a relatively large village forming the base-camp. The vihage of Mangindrano (WGS84 UTM coordinates: E279684 N8421586), at the southern foot of the 'Massif de Tsaratanana' (not to be confused with the tectonic 'Tsaratanana Sheet') was chosen as base-camp from where the geological data was collected and processed. Only during 10 days was this base-camp moved by means of an expedition over the scarp of the first mountain chain to the NE, where data was collected along the river the 'Bemafo' draining through the jungle in a SE direction. The fieldwork was carried out during September and October 2005, which coincides with the dry season in the north. The wet season namely makes this region inaccessible, especially by car. The collection of data in the field was done with the help of a global positioning system (GPS: Garmin), the l : 100 000 maps, a miniature version of the geological 1: 500 000 map (Besairie, 1971)) and additional conventional field equipment. The processing of the data was subsequently done by plotting different types of data (e.g. foliation- and stretching hneation measurements) on transparent paper overlaying geographical constructed map on mm-paper. Furthermore, stereographic projection plots were made of the different data using a laptop and the plotting software GEOrient 9.2 (Holcombe, 2005).
5.2 Geology a n d Morphology
MULDER 14
Tsaratanana Maxsif
: Sambirano-Sahantana Group - Gneiss mica schists ; Sambirano-Sahantana Group - Quartzites : Granite sti"atoids
: Rhyolites and Trachytes (Tertiary) I : Basalts (Tertiary)
: Alluvial sediments (Quarternary)
Figure 6 - Detailed geological map ofthe field reseai'ch area (Besaiie, 1971) with Sl orientations.
become mountains and their appearance becomes more rigid with the vegetation density increasing significantly (cover > 90%). Despite the decrease in physical weathering, the chemical weathering and the density of the cover inhibits any data to be collected here. The only locations with reasonably fresh outcrops are along the bedding of the numerous rivers and creeks that have incised themselves in the bedrock of the 'Massif de Tsaratanana' and drain the basins to its south. The collected data are therefore spatially biased due to the fact that they only capture the geology along the rivers and streams and not within the hills and mountains. The sub-area on the other side of the first mountain ridge to the NE, did yield some spatial variation but still consisted of data along a river.
5.3 Lithology
mylonites have been observed at several locations (fig. 7a) and are easily recognised by their ductile appearance and strong foliation in combination with a very fine grained structure. The mylonites do not seem to have a different mineral composition and are therefore only structurally different.
Fig. 7 - Different structures found in the research area: (a) mylonitic Outcrop describing Sl; (b) granitoids (G) cutting through gneiss rock and its Sl; (c) folded leucosome (Leuc); (d) granitoids (G) clearly cutting through fohation (Sl).
MULDER 1 6
of granitoids within the host gneiss varies from virtually grainitoid-free to pure granitoid without any country rock gneiss. The leucosomes (fig. 7c), having a thiclmess of mm to cm-scale, are especially distinct in the massif darker amphibolite-like rocks and are mostly deformed into small open and asymmetric folds. Their mineral compositions could not be determined in outcrop, but these leucosomes are presumably concentrated layers of quartz and plagioclase.
The granites in the area, which are found at the mountain scarps, were of the same mineral composition as the gneisses, namely quartz, feldspar, biotite and hornblende, but hardly contain any planar fabrics or other deformational stractures. These granites are referred to as rhyolites/trachytes of Tertiary age (fig. 6) by Besarie (1971).
5.4 Petrology
On the basis of their mineral assemblages, the samples have been devided into three sub-groups. The first group mainly contains:
Hb + Plag + Qtz 4- Bt
Hornblende (30-75%), plagioclase (25-40%), and quartz (10-30%) are dominantly present where biotite is less abundant (< 10%). Further accessory minerals are apatite, titanite and very little microciine (< 5%). The second group has a mineral assemblage as follows:
Plag + Qtz -t- Bt Hb
This group is especially characterised by the absence or the very little presence of hornblende (<20%) compared to the other samples. The mineral assemblage further contains: plagioclase (30-50%), quartz (15-35%) and biotite (10-35%), in combination with minor presence of microchne, titanite and apatite (< 5%). The last group has the mineral composition of:
Hb + Plag + Qtz + Bt + Ep + Chi This assemblage is specially marked by the presence of epidote (< 30%) and chlorite (< 5%) in smah to moderate amounts in combination with hornblende (20 - 75%), plagioclase (10-40%), quartz (10-40%) and in lesser amounts biotite (< 15%). Further minerals that were observed in smaU amounts are microchne (< 5%), titanite (< 5%) and apatite (< 15%).
Concerning the spatial distribution of these three mineral assemblages, no relations could be found especially with regard to the epidote and chlorite containing rocks. The hornblende-bearing and the hornblende-free roclcs seem to show a clustering along the main river and its distributaries Ijdng to the north-east of Mangindrano. However, the dataset is not large enough to ascertain this distribution.
5.5 S t r u c t u r a l F e a t u r e s
Fig. 8 - Fold types found in tlie research area: (a) recumbent and closed fold; (b) asymmetric fold; (c) open folding.
enough measurable foliation which were used in making a surface map and calculating large scale possible folds. Only one generation of foliation could be found, which wih be referred to as Si from now on (fig. 7a). Si was formed during the first deformation phase (Dl) observed in the research area. The orientations of the Si are presented spatially in figure 6.
MULDER 1 8
Fig. 9 - Open folding (F3) deforms closed and recumbent fold (F2).
asymmetric folds measured and calculated fold axes have a mean orientation of 230/15 (Fig. lie) with a variance smaller than the other axis plots, namely 0.44 with a dataset of 15.
The structural relationships between the different folds clearly show that the open folds developed latest, deforming the other folds (fig. 9). The tectonic interrelationship of the asjmimetric and the closed and recumbent folds was not clear, also due to the fact that the asymmetric folds are not an independent sub-group because most of them were also closed and recumbent. Therefore, these two fold types are taken as one group (Fa) and therefore associated wdth one deformation phase (D2). The subsequent deformation phase, causing open folds (F3), is therefore referred to as D3. The asymmetric folds are ubiquitous, well developed and often complex due to their 3D-components. These asymmetric folds were consequently used as shear sense indicators. The results are, unfortunately, not consistent and therefore qualitatively useless to the research.
lo). The relative timing of the event that caused the lineations could not be defined in the field. The stretching lineations are mostly found on the fohation surfaces of the gneisses and rarely on the granitoid surfaces. The spatial variation is, however, large and so is the data set (70). The mean direction of all of the measured stretching lineations is 087/04 with a variance 0.59 (fig.iid). The very high variance could be induced by measurement errors but also by variation in paleo-strain or a later deformation phase.
Further remarkable but not quantitatively used structures included tectonic breccia's, rotated clasts, boudinage and presumable 'shear' zones. Shear zones were observed in two cases only and this interpretation was supported by mylonitic textures of some outcrops (fig. 7a). Unfortunately no analytical techniques could be applied to the concerning structures. This in combination with the lack of a meaningful spatial distribution, the contribution of the structures was hmited to giving a geological impression of the study area.
5.6 U n s t r a i n i n g of D 3
Due to the fact that the open folding influences most other structure in the area, it can be assumed that the a-symmetric and the closed and recumbent folds and their corresponding axis where also influenced by the open folding. Rotating the mean fold axis of the a-sjmimetric and the closed- and recumbent folds along the mean open fold axis would then remove the effect of the open folding and hopefully enhance the clustering of the other folds axis. The fold axis was rotated for 40° establishing a minimum variance.
In table 1, the rotated fold axes are shown in comparison to their non-rotated counterparts. Despite the fact that the directions of the fold axis changes little, there is a
Direction Az3Tiiuth Variance Data set Asym.fold axis (rotated) 224 42 0.37 15
Closed- & Recumb. folds (rotated) 245 47 0.25 31
Asym.fold axis (not rotated) 230 15 0.44 15
Closed- & Recumb. folds (not rotated) 246 12 0.46 31
Table 1 - The non-rotated compared to the rotated mean fold axis orientations of the a-symmetric folds and the closed- and the recumbent folds.
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6. Lattice-Preferred Orientation of Quartz
6.1 Introduction
Lattice-preferred orientation (LPO) refers to not randomly distributed lattice-orientations of the crystals of a rock, which is often the case in deformed roclcs (Passchier & Trouw, 1998). For minerals like mica's, this preferred orientation is often easily recognised by their planar and elongated grain shapes. However, in the case of quartz this is not the case and further analysis is needed to determine if any LPO is present. For equant grain shape minerals lilce quartz dislocation creep is believed to be an important deformation mechanism (Lister & Price, 1978; Passchier & Trouw, 1998). In a random oriented group of quartz grains, the crystal axes of the grains will rotate during deformation with respect to the instantaneous stretching axes (ISA) giving them a preferred orientation. The fohowing factors are important concerning the type of LPO created: 1) the slip system that is active and the amount of activity on each slip system; 2) The ratio of stretching rates along the ISA of the flow, determining the direction the crystals will rotate and therefore the shape of the fabric; 3) The finite strain; 4) The Idnematic vorticity number; 5) The activity of dynamic recrystallisation: in this case LPO may increase or decrease; and 6) Growth of grains from solution which can produce preferred orientation (Passchier & Trouw, 1998). The best understood factor, however, in the development of a LPO fabric is the slip system. With help of, among others, theoretical and numerical modelling of fabric development, it is now possible to determine the slip system by looldng at the pattern of the measured a- and c-axes in a stereogram. Because the type of slip system active is dependent on their critical resolved shear stress, the metamorphic and deformation conditions can also be derived (Passchier & Trouw, 1998). Depending on the slip system, the c-axis plots that appear in the stereograms can form certain pattern lilce clusters, great circle girdles or crossed girdles (Passchier & Trouw, 1998). In this study, two quartz rich samples are analyzed to investigate any lattice preferred orientation, with the aim to elucidate some detaUs of the deformation history.
6.2 L P O patterns, some theoretical background
MULDER 22
namely the shear plane, the orientation of the stretching axes but most importantly the direction and shear sense of the simple shear deformation it has undergone (Lister & Price, 1978). When dealing with a c-axis fabric that shows a maximum, the pole free areas indicate the stretching direction whereas the location of c-axis maxima indicates the contractional direction. This information in combination with the foliation, gives the shear sense of the system (Lister & Hobbs, 1980). As long as the deformation type (e.g. simple shear) prevails, the pattern elements that define the fabric skeleton will not change. There are however restrictions when interpreting the shear sense according to asymmetric c-axis fabrics. It needs to be taken into account that the fabric derived from the c-axes is associated with the last stages of deformation. Due to the fact that the fabric skeleton may alter quicldy when the deformation type is changed, the former deformation pattern can get lost (Lister & Price, 1978; Lister & Wilhams, 1979; Lister & Hobbs, 1980). However, because the main orientation of the fabric does not change considerable, a shear sense can still be derive when using intensity distribution (contour) plots (Lister & Williams, 1979). Furthermore, a factor in determining the final fabric is the starting grain population. If this has been altered by an older deformation, this can have consequences for the derived fabric and therefore needs to be taken into account during interpretation (Lister & Williams, 1979). Like in any other experiment, multiple sample examination is of course essential if one wishes to arrive at meaningful conclusions. Especially when dealing with a shear zone it also has to be taken into account that despite the fact that bulk deformation is non-coaxial, coaxial regimes can be active at a smaller scale within the shear zone (Lister & Wihiams, 1979) maldng multiple samples essential in a research.
6.3 Methods
First, a selection of potentially suitable samples was made by microstructural analysis of the sections. Furthermore, for the illustration purposes a Leica DEC 500 digital camera system situated at the University of Utrecht was used in order to produce microscopic images. The main criterion for the selection of the samples was a high concentration of quartz grains so that at least 100 c-axes could be measured and the influence of surrounding grains was minimized. Furthermore, the quartz grains had to be large enough (> 30|im) for the use of a microscope. After selection, the samples were mounted on the universal stage (U-stage) and consequently the optical axis of the quartz grains were measured according to the methods described by Turner & Weiss (1963). Due to the measurement of only one type of crystal axes (Passchier & Trouw, 1998), the orientations in comparison with the foliation were plotted on the lower-hemisphere of an equal-area projection. Due to the fact that the preferred-orientation was weak and therefore difficult to notice in a stereogram, pole-free areas and contour plots were used to enhance the pattern. The contour plots which followed Kamb's formula (Kamb, 1959) and where the deviation (sigma) was used as a unit for the contour lines, were processed using the software Stereonet 3.02 (Steinsund, 1992-1995).
6.4 Microstructural A n a l y s i s
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sample are titanite (<5%), microciine (-1%) and finally apatite (<i%). Samples 12-9 consists mainly of plagioclase and quartz (both 30%), with smaher amount of biotite (~15%). Other observed minerals maldng up less then 5% of the assemblage, are apatite and microciine.
The microstructural features of this sample 19-6, are dominated by the mineral morphology of the constituent major minerals. Without a microscope the thin-section displays a clear foliation marked by the irregular alteration of green/darker minerals and transparent bands (0.1-0.5 mm). Under the microscope, a granular appearance is noticeable with darker bands dominated by hornblende and biotite, where the lighter bands consist mainly of plagioclase and quartz grains (fig.i2a). The biotites are the smallest of the major minerals (<imm) and are easily recognised due to their characteristic columnar/fibrous minerals, mostly present as aggregates, i n combination with their typical brown-green colour when looking through plain and crossed-polarized light. The green hornblende are larger (i-3mm) have a high relief, clear crossed cleavages (56° and 124°) and are very irregular but often with straight crystal faces (fig.i2a,b,c). The hornblendes also display clear triple-junctions and are shghtly elongated parahel to the main foliation (fig.i2b). The plagioclase (~o.5mm) is identified mostly by its euhedral shapes and the vitrous appearance and twinning induced by stress and certain temperature/pressure conditions. The twinning is sometimes mechanical (fig.i2c) and sometimes in the form of growth twinning (figi2d). The quartz grains in this sample are sometimes present as characteristic elongated grains, running through the sample as ribbons (up to 5mm)(fig.5d), but mostly as subhedral grains (0.5-imm) which lack cleavage in combination vdth a low relief. Undulatory extinction is often present concerning the plagioclase and the quartz grains making the measurement of the latter c-axes difficult. The second sample studied, namely 12-9, does not have a distinctly fohated appearance, also due to the lack of variation between coloured and transparent minerals. The majority consists of transparent and light minerals, with some strands of green/dark coloured grains. Using the microscope a gradation of grain size is observed going from one side to the darker regions of the thin-section. The hghter region has a very granular
appearance and consists predominantly of quartz and plagioclase minerals with an average grain size of <o.5mm (figi2e). The plagioclase twinning is not weh defined in this region, except a couple of growth and mechanical twins. The darker side of the thin-section is characterized by a large variation in grain sizes. The quartz (ribbons) and plagioclase of i-3mm are found, which are mostly parallel to each other and form the a slight foliation. Plagioclase grains often show undulatory extinction and sometimes growth twins and sporadically mechanical twins. Furthermore the biotites, which are also aligned along the foliation, make their entrance becoming not larger then 0.5mm (figi2f).
6.5 C-axis Measurements
Figure 13 shows the LPO contour plot of quartz c-axes from sample 19-6 and 12-9 with the fohation shown horizontal. The plot from 19-6 shows a c-axis maximum sub parahel to the fohation whereas the plot from 12-9 displays a mirror image only with two slight maxima sub parahel to the foliation.
7. Monazite Dating
7.1 Monazite O c c u r r e n c e
Monazite [(LREE) PO4] is a hght rare earth element (LREE) bearing phosphate containing large amounts of thorium (Th) and uranium (U). Due to its crystal structure, monazite does not incorporate lead (Pb) initially at growth, but radioactive decay of Th and U does create radiogenic lead which consequently increases with time (Foster & Parrish, 2003). The closure temperature (Tc) of monazite is dependent on many factors like duration of heating, cooling rate and crystal size and is therefore characteristic for each monazite (Suzuki et al., 1994; Smith & Giletti, 1997). However, the closure temperature is often assumed to be in the range of 700-750°C (e.g. Townsend, 2000). Monazites are mostly found in felsic rocks like metamorphosed granites and pelites, and igneous roclcs (Overstreet, 1967; Parrish, 1990), but can, although less frequently, also be found in some mafic and calcic roclcs (Spear & Pyle, 2002). Upper greenschist grade metamorphism is needed for the formation of monazite, however, the exact temperature is determined by the bulk composition of the rock (Foster & Parrish, 2003). Due to the resistivity to weathering, monazite also occurs as detrital grains. However, it is often not present in lower-grade metamorphic roclcs due to its presumable reactivity during diagenesis (Parrish, 1990; Spear & Pyle, 2002). Further constraints on the formation of monazite can be found in Spear & Pyle (2002).
7.2 U - T h - Pb dating w i t h monazite
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subsequently dated in a monazite (Parrish, 1990; Suzuld et al., 1994; Spear & Parrish, 1996; Smith & Güetti, 1997; Crowley & Ghent, 1999; Aleinikoff, 2000; Foster & Parrish, 2003). Electron Microprobe Analysis (EMPA) is one of the methods which can estabhsh the needed spatial resolution (i-3|im). Due to this and additional advantages outlined below, EMPA was the means of analysis in this research.
7.3 E l e c t r o n Microprobe
The use of the electron microprobe for monazite dating is possible because, as opposed to isotope measurements, the lack of initial Pb makes oxide measurement sufficient for dating. Additional advantages to the electron microprobe dating are that samples are not damaged during analysis, the preparation time and the analysing process are quick and, in addition, also relatively cheap. Furthermore, the analysing technique is in-situ, meaning that the monazites can be measured without extracting them from the sample. The consequent advantage is that the exact location of the monazite can be retrieved and therefore the interaction between its surrounding can be taken into account. Additionally, the microprobe has a very high spatial resolution of about 1-3 |im (Suzuki et al., 1994; Montel et al., 1996) and can therefore identify very slight zoning, maldng the chance of a discordant age smah. Electron microprobe analysis is based on X-ray emission that occurs when an electron is accelerated leaving the inner shells of the atoms nucleus. This electron jump that fills the vacancy will generate an X-ray that is emitted and which has a measurable wavelength characteristic for the element. The amount of each component is consequently derived by the measured intensity of the X-ray (unpubhshed work: Haring, 2005). Due to the fact that per element several types of rays can be emitted, specific X-rays are taken for the analysis of different elements depending on the possibihty of interference. Concerning monazite dating, the X-rays that are best analysed are UM(3, Th M a i and PbMai (Suzuki et al., 1991; unpublished work: Harings, 2005). The errors, possibly occurring during analysis, can be machine-related and sample related and are explained in detail in Pyle et al. (2002 & 2005).
7.4 Methods - E l e c t r o n Microprobe A n a l y s i s ( E M P A )
detection System (EDS), five Wavelength Dispersive Systems (WDS) detectors, a Secondary Electron (SE) detector and a Backscatter Electron (BSE) detector. First the monazites were detected in backscattered mode (BSE), in which brightness of the grains is determined by the mean atomic number of the minerals. After the monazites were detected, the BSE was used to construct an image showing the minerals with a high atomic number as bright and those with a low atomic number as being dark. Two images per monazite were made, namely one using a high magnification and focussing on the monazite grain and the other with a lower magnification capturing the monazite's environment. The high magnified images of the monazites were used to determine the location for the spot analysis so that no ahochtonous inclusions or neighbours are taken into account. After the coordinates corresponding to the monazite locations were derived, the associated element amounts of among others the LREE, PbO, UO2 and ThOa (wt%) on these locations were measured by the WDS. The WDS analysis was performed at constant conditions with the accelerating voltage set at 20kV and a beam current of sonA. Further detaUs concerning the conditions during analysis are explained in unpubhshed work of Wiggers de Vries (2003) and Harings (2005).
7.5 Age Calculations
The basis on which the monazite dating calculations have been developed is the decay scheme of the U-Th-Pb system (Faure, 1986). In combination with the decay constants (X238 = l.55i25'io-io yr\ X235 = 9.8485-10-1° jr\ X232 = 4-9475-l-0-^° derived by Steiger & Jager (1977) and the cumulative radiogenic Pb generated, the apparent age (t) can be calculated:
_ totalp}) - initialpl) + 232Th(e^(^33Th)t - l ) + 235U(e''(^35U)t - l ) + 238lJ(eX(238U)t _ ( i ) Applying the present day 238U/235U (Steiger & Jager, 1977), discarding the initial lead and replacing the element amount by oxide amounts, the equation fohows (Suzuki et al., 1991):
PbO ThO, _^0,r(l37.88..^'^^^^"+.^'^"^>') ;
V 138.88
(2)
Using the measured elements (PbO, ThOa and UO2) and the molecular weights of the oxides (WTh = 264, Wu = 270 and Wpb = 224), the apparent age (t) is derived assuming that the apparent age associated with each decaying element is the same. Due to insufficient number of measurements, no cumulative probability plots had to be made.
MULDER 28 - ThO* =ThO^ + ,X(ThH2)l UO^ • 264 -l)-270 V (137.88 •e^'^"^"+e^<^^^'^>' 138.88 - 1 J (3)
Since the equal age points in an ideal situation plot on one line, the Concordia wih be described by means of a linear equation, where the slope is related to the isochron age by means of the following equation:
In a non-ideal situation, the data points wih not perfectly lye on one hne, therefore using regression a best fit line is constructed. This implies that the calculated isochron age wih have an error described by the distance of the points to the Concordia.
7.6 Results
Out of the eight samples that have been prepared for the EMPA, only three could be used for dating due to a lack of time and the availability of measurable monazites. The three thin sections that were analysed are all from granitoid rocks and contained groups of monazites remnants maldng them very small (<iO|nm)(figi4). From the thin sections, 7 monazites were identified of which subsequently 23 spot analysis were done (table2). Of these spot analysis two were damaged and consequently not taken into account further in this research, and of the rest only one measurement (9-i(I)-2a) achieved a sufficient total compound wt%, namely of >107%.
Despite the fact that the individual measurements could not be used to derive valid apparent ages, the PbO content may stih give an indication of the geological period in which they were formed. Subsequently the 19 remaining analyses were plotted in a Th02* vs. Pb diagram (fig.15) in combination with isochrones of 500 Ma and 2.5 Ga as reference, indicating the period embracing ah metamorphic from northern Madagascar established to date. Using the diagram with the plotted measurements, three sub-groups were made on the basis of their apparent ages. One subdivision of 11 measurements indicates apparent ages of larger than 4.0 Ga and was therefore not taken into account further in this research concerning their age indication. Two measurements indicated apparent ages of around 3.0 Ga, faUing outside the reference isochrones and therefore forming another sub-group. The final sub-group was made ofthe remaining six measurements, having the interrelationship that they aU faU into the period of 2.5 Ga and 500 Ma. Of this last sub-division an isochron was constructed associated with an isochron age of ~1.3 Ga.
8. Discussion and Conclusions
8.1 Implications of the Petrology of the Study A r e a
According to the geological maps, originally by Besarie (1971) and later modified and further specified by Buchwaldt et al. (2003)(fig.6), the study area is sUuated in the central part of the Sambirano - Sahantana Group, which is assumed to be the suture zone between the Bemarivo Belt and the Antananarivo and AntongU Block. According to Buchwaldt et al. (2003) this part was identified as consisting of metapelites and pelitic migmatite rocks like quartzite, mica-schist, aluminous gneiss, and lesser
MULDER 30
comp wt% PbO ThOa U02
9-i(I)-Ja 65 0.81 8.807 1.886 9-i(I)-i& 70 1.236 1.48 0.314 9-i(I)-Jc 83 1.56 1.648 0.297 9-i(I)-Jd 82 1.652 1435 0.259 9-i(X)-ie 78 1.602 1.675 0.295 9-l(I)-2a 107 0.766 1.089 0.168 9-i(I)-2Ö 91 1.06 1.058 0.221 9-l(I)-2C 85 0.863 1.089 0.181 9-i(I)-2d 89 0.63 0.642 0.164 9-i(I)-2e 85 0.779 0.582 0.175 9-i(I)-2/ 92 0.627 0.943 0.183 9-l(lï)-la 85 0.238 2.054 0.146 9-i(II)-i& 85 0.195 2.075 0.165 9-i(II)-2a 90 0.732 2.875 0.166 9-l(II)-2& 89 0.575 3.213 0.179 9-l(II)-2C 87 0.576 3.369 0.213 ll-2-ia 83 0.4 4.988 0.164 11-2-1 b 86 0.364 5.641 0.204 11-2-lC 84 0.448 5.166 0.174 ii-2-2a 50 0.386 4.923 0.149 11-2-2& 52 0.362 4.86 0.16 ii-2-3a 42 0.229 2.639 0.102 11-2-3& 38 0.231 2.096 0.8
Table 2 - All the WDS measurements and the consequent total compound weight percentages with the PbO, ThOa and UO2 amounts.
EMPA results of all Granitoid IVfonazites
1.80 1.60 - 1.40-1.20 1.00 • 0.80^ • 0.60-0.40 -0.20 -0.00 J aoo 5.00 laoo 15.00 20.00 25.00 • 500 [Vb 2,5 Ga • >4.0Ga m ~1.3Ga A ~3.0Ga Linea'r (500 Ma) Lineair (2,5 Ga) Lineair (~ 1.3Ga)
marble. The petrology of the samples taken in this study around Mangindrano contradicts this. Microscopic analysis shows that the gneiss samples in this research contain hornblende and epidote, and that they lack sihimanite and cordierite. This consequently indicates that the original rocks were most probably granodioritic in composition, which therefore indicates that they are orthogneisses. Furthermore, Buchwaldt et al. (2003) state that the eastern part has undergone granulite facies, as opposed to the orthogneisses found in this study, which experienced most probably only upper-amphibolite (Janssen, 2005). However, this does not take into account that a regional metamorphic gradient can be present in between the two field work areas. Janssen (2006) suggested that the difference in metamorphic grade could be explained due to a relatively deep position of the metapelites in the Lokoho and Sambirano region with respect to the orthogneisses in the Mangindrano region during peak metamorphism, however this could not be supported by evidence found in the field. A more probable link can be made with the western part of the Sambirano-Sahantana Group, namely the tonalitic to granodioritic gneisses (882 Ma), amphibolites (-575 Ma) and dikes/sheets of pink leucrogranites (~56o Ma) cutting the host lithology (Buchwaldt et al., 2002). This is especially remarkable because the pertinent pink leucogranites seem very similar to the pink granitoids found in the Mangindrano area. I t is not clear from the hterature where the transition between the east and the west of the suture zone is located, however the lithologies examined by Buchwaldt et al. (2002) were found along the Sambirano and the Mahavavy rivers which he within a radius of 10km from the Mangindrano research area. A discrepancy, however, between the lithologies of these two regions, is that the orthogneisses in western Sambirano have a subduction signature (Buchwaldt et al., 2002) as opposed to the orthogneisses of the Mangindrano area (Janssen, 2006). Another possibhity, which is shghtly less likely but cannot be excluded at this stage, is that the orthogneisses could belong to the rocks of the Mananbato-Base Group. Despite the fact that no leucrogranites or granitoids are found, this group does contain similar granodioritic gneisses of amphibolite metamorphic grade extruded at ~750 Ma and subsequently metamorphosed at 5ii± 5 Ma (Buchwaldt et al., 2003). However, according to the accepted geological map of Besarie (1971) and Buchwaldt et al. (2003) these granodiorites he at least 20 km to the north of the research area around Mangindrano (fig.6). Valid dating of the orthogneiss rocks could have given a strong link between western part of the Sambirano-Sahantana or the Mananbato-Base Group, however due to the lack of rehable results and the deviating age indications, no further suggestions could be made during this research. The implications of these findings consequently concern the location of the thrust zone only. According to the current geological maps of the north (Besairie, 1971; Buchwaldt et al., 2003) the thrust zone is defined by the Sambirano-Sahantana Group. If the orthogneiss roclcs found in this research are part of the Mananbato-Base Group then this would alter the boundaries of the contact zone. However, when linldng them to the western part of the Sambirano-Sahantana Group then this would only alter the subdivision within the suture zone. It is suggested here that the boundaries of the different formations within and around the suture zone need to be specified and/or changed to implement the findings of this research, but support by further petrological and geochronological research is needed.
8.2 Implications of the s t r u c t u r a l history of the study a r e a
MULDER 32
not be examined in further detail. The isoclinal folds subsequently deformed the Si due to a presumably NNW-SSE directed compression. This could have been a two-step-deformation, involving first the development of open folds and then, due to shearing, the development of isoclinal and recumbent folds. Finally, influencing all the other structures, open folding was induced by an EW compression (D3). In this order it is stih uncertain when the observed stretching lineations were formed. The mean direction of the stretching lineation, namely 087/04 (fig.iid), as compared with the mean orientation of the fold axis of the different folds, suggest that the most probable deformation causing this lineation would be D3. This is based on the observations that the mean open fold axis lies normal to the mean direction of the stretching lineation. On the other hand, the removal of the effect of D3 augments the clustering of the measured stretching lineation giving evidence that the D3 influenced a pre-existing stretching hneation (fig.iid and e). Furthermore, it does not seem probable that a deformation regime leading to open folds only would give rise to any stretching lineation. Stretching lineations are often associated with shearing (Passchier and Trouw, 1998) and therefore could more easily be linked to the D2 which formed the isoclinal folds. If it is assumed that the isoclinal folds were formed during a progressive two steps history, the stretching hneation could be linked to the shearing which turned the open folds into isoclinal folds. This could explain why the mean fold axis of the isoclinal folds is not perpendicular to the mean direction of the stretching lineation. The mean fold axis in this case would be a result of a progressive deformation involving shearing at a late stage.
to explain. Taking the NW-SE oriented Sambirano-Sahantana Group as the suture zone in combination with the E-W oriented stretching hneations, a transcurrent movement seems a more likely option. Especially in combination with the sub-horizontal fohation found in our study area, a transtensional regime could be a possibility (Wicldiam and Oxburgh, 1985 and 1986). The lower PT-conditions linked to such an event seems to be more adequately explained by continental rifting than by any event related to juxtaposition of the Bemarivo Belt and the Antananarivo and Antongil Blocks. However, relative or absolute timing of the stretching hneation are needed to link this inferred transcurrent event to the different types of structures found in the field. At this moment it seems plausible that a transcurrent event linked to the stretching hneations was older than the peak metamorphic event, because the (partial) recrystahization most probably induced by the peak metamorphic event shows little to no strain and additionally overprints ah deformational structures on a micro-scale. I f this event, a probably continental rifting, caused the peak metamorphic event observed at the Lokoho region then this would mean that the continental rifting occurred at -520 Ma and consequently maldng the transcurrent event >520 Ma. Unfortunately, no clear peak metamorphic event was recorded in the west of the Sambirano-Sahantana Group. It fohows that without reliable dating of the orthogneisses in the Mangindrano area, no definitive conclusions can be drawn concerning the (relative) timing of the peak-metamorphic event. However, according to the findings of Janssen (2006) the geotherm of the Mangindrano orthogneisses are similar to those found in the Lokoho and the western Sambirano-Sahantana region. This, in combination with the simüarities in lithology between the latter and the Mangindrano area, gives support to a coeval peak-metamorphic event in the suture zone at around 520 Ma, which places the amalgamation of northern Madagascar at some time before.
MULDER 34
on the rocks that possess the hneation and those that do not. Furthermore, the different folds found in our area could partially be correlated to other folds found by other research projects. Buchwaldt et al. (2003) found isoclinal folds in the eastern part of the Sambirano-Sahantana Group, however no axes measurements were reported which makes the correlation difficult.
8.4 Implication f r o m L P O i n quartz
The quartz c-axis plot of sample 19-6 shows two maxima almost parallel to the foliation and therefore can be seen as an asymmetric pattern corresponding to shear deformation (Lister & Hobbs, 1980; Passchier & Trouw, 1998). The derived shear sense, by looking at the pole free and the maximum areas (Lister & Hobbs, 1980), is dextral (fig.i6). When putting this in context with the orientation of the sample 19-6 in the field, the dextral shear sense implies a top-to-the-west movement, assuming that the sample is at the hanging wall of the suture zone. The quartz c-axis plot derived from thin-section 12-9 displays a slightly asymmetric pattern also implying a shear deformation. However, in this case the shear sense was interpreted as being sinistral (fig.i6). Only when looking at its original position in the outcrop it appears that despite the different shear sense, 12-9 also displays a top-to-the-west movement. These results fit the fieldwork stretching lineation data very well. Due to the fact that shear movement was extensively witnessed in the field by means of asjmimetric folds and stretching lineations, the shear development of the asymmetric quartz c-axes fabrics is therefore also most likely linked to the same movement. The orientation of the stretching lineation supports this because its mean orientation has been calculated to be ~EW-trending. Taldng into account the orientation of the sample, this would imply a top-to-the-west movement which is concordant to the stretching lineation found in the area. More specifically on the basis of the type of asymmetric pattern, another interesting inference can be made concerning the ambient temperature during flow. According to Passchier & Trouw (1998), a c-axis maximum, as opposed to asymmetric girdles, corresponds to (very) high temperature deformation which is supported by the petrology and thermometry research on these orthogneisses by Janssen (2006). Despite the good fit of the LPO results with the observed lineations, cautions needs to be taken and further research is needed in order to relate the derived shear sense to a regional event. Local deformational regimes can differ from a regionally one, making the need for multiple samples analysis a necessity (Lister & Williams, 1979). Due to limited time for this study no further LPO analysis was made, but this would consequently be a suggestion for further research.
8.3 Implication f r o m Geochronology
significance of the resuhs, because in the field it was concluded that the granitoids were younger than most of the structures. This implies that i f an age is derived this could be used as a link to the leucrogranitoids found in the west (Buchwaldt et al., 2002) but due to the lack of deformational structures in the granitoids, it would be doubtful i f it would constrain the age of the tectonic event which caused the suture zone. Unfortunately, due to the bad quality of the monazites, no valid results could be obtained which consequently means that no hard evidence is avaUable from this study to test the various hypotheses. However, because Pb is not easily lost during the EMPA, even a failed analysis would give an indication of the amount of Pb in the monazUes. Because it is assumed that ah the Pb in a metamorphic monazite is radiogenic, this would imply that, despite the fact that a vahd apparent age could not be derived, the high PbO measured indicates that the monazUes have to be very old. This assumption, however, stUl bears some major uncertainties. First of ah, 11 of the 19 measurements used, yield such a high PbO content that apparent ages of > 4.0 Ga were reached (fig. 15). This unreahstic result makes the use of Pb content as an indication of age very questionable. Furthermore, the remaining 8 measurement gave ages between 3.2 and 1.3 Ga which is remarkable giving the fact that no roclcs of that age range have been found in this suture zone or elsewhere in the Bemarivo Belt, thus making the correlation very difficult if not impossible. The nearest rocks of a simUar age are the Precambrian rocks of the Antananarivo Block (Collins, 2006) which would imply that the Mangindrano area is not part of the suture zone but belongs to the Antananarivo Block, which consequently puts the study area in the footwall ofthe thrust bringing the suture zone more to the north. Imphcations concerning this theory would be that the research area was part of the passive margin of the Antananarivo mainland and consequently became the footwall of the apparent thrust that fohowed. This hypothesis does not seem viable, given the fact that no metapelites nor any other evidence for a passive margin were found in the area. Additionally, the monazites that gave these high PbO values were very abraded as can be seen on the BSE images (fig.14), which adds to further uncertainty about the derived ages. Most of the monazites that were found were present in groups of smaher monazite, giving
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evidence for microstructural instability. Because the monazite groups fall within a larger former mineral structure (fig.i4b and c), it does not seem improbable that the monazite were initially large but became unstable, possibly dissolving into smaller remnants. Even if this was the case, this would not explain the high lead level because research concerning this topic only mentions cases of the loss of Pb and not the incorporation of Pb. Cases of Pb inheritance in zircons are often observed giving much higher discordant ages. However, in the case of monazite, inheritance is rarely documented and the process, if it occurs, is poorly understood. Copeland et al. (1988) derived older monazite ages from a Tertiary granite in the Himalaya's and suggested that this was due to the mixing of young monazite and an older inherited component. A case similar to this was found during the research of Mougeot et al. (1997) and Cocherie et al. (1998). Mougeot et al. (1997) observed Pb inheritance in abraded grains in granitic migmatitic orthogneiss roclcs of the Central Massif, consequently giving older ages than the metamorphic event. Because metamictisation was active in that area, Mougeot et al. (1997) could only suggest this as a source for the high Pb levels in monazite, but he did not further investigate this possibility. Cocherie et al. (1998) witnessed Pb inheritance in monazites in migmatites from the Ivory Coast. Random sub-microscopic blotchy patches varied up to 700 Ma in age and where explained in Copeland et al. (1988) by mixing which could be witnessed as zones. These cases do seem simhar to the discordant monazite ages in this research but lack of detaUed study precludes further comparison. The in-situ monazites on the BSE images (fig.14) seem to be unstable, but it is not clear from the literature if the monazites with inherited Pb also display this texture.
Unfortunately, the identification of Pb inheritance does not give additional constraints on the deformation history of the granitoids, due to the lack of information concerning this process. Concerning the instability of monazites, possible implication could be derived. Accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism (Finger et al., 1998). As opposed to metapelites, this is something which is observed in granitoids only. The break-down process, on the other hand, implies the formation of apatite-allanite-epidote coronas, which have not been observed in this research. Monazites do, in some cases, contain some neighbouring minerals with lower atomic number, indicated by the black colour. In one case in this study the chemical analysis gives a spectrum with high C, Al and Si peaks which could indicate the of presence ahanite. Finger et al. (1998) furthermore states that during the break-down the PO4 tetrahedra mineral structure is maintained. In the BSE image (fig. 14b and c) monazite appears to be a remnant within its older structure possibly indicating that the mineral structure is maintained despite the break-down of monazite. Unfortunately, no chemical measurements were made of the mineral making up the rest of the structure besides monazite. A requirement for the initiation of the break-down process at amphibolite grade metamorphism is the availabihty of a metamorphic fluid phase which delivers the needed components to the monazite grain boundaries. The final controlhng factor for the replacement of monazite is the diffusion rate of the elements that are introduced and diffuse (Finger et al., 1998). In combination with the high Pb content of the monazites, this could imply that at the monazites in the granitoid were altered due to a metamorphic fluid which could also have mixed the young monazites with older components consequently increasing the lead content.
indication for the apparent age of the granitoids. On the other hand, the abraded monazite structures found do possible indicate break-down of the monazite due to its presence at the amphibolite metamorphic grade. Thermobarometric research of the orthogneiss rocte cohected during this project by Janssen (2006) have derived temperatures of 695-776°C at pressure of 4.i9-5.5ikb which is consistent with an amphibohte facies metamorphic grade. Furthermore, the mineral assemblages in the orthogneisses do indeed display amphibolite grade metamorphism, with a fmal retrograde path.
8.5 Conclusions
Conclusions that can be drawn from the different lines of research undertaken on the tectonic contact zone between the Bemarivo Belt and the Antananarivo- and AntongU Blocks are as follows:
o The occurrence of orthogneisses rocks in combination with invaded granitoids found in the Mangindrano field area imphes that the formation boundaries as indicated by the maps of Besairie (1971) and Buchwaldt et al. (2003) need to be further specified, and possibly modified such as to alter the position of the contact zone of the Bemarivo Belt with the Antananarivo Block,
o Field study result and LPO analysis of quartz rich gneisses support a shear deformation with a WSW-ENE trend and a probable top-to-the west shear sense which is at variance with the presumed top-to-the south shear sense linked to the Bemarivo thrust.
o LPO patterns of quartz furthermore lend support to the (very) high methamorphic temperature conditions found by Janssen (2006).
o Thermobarometric data (Janssen, 2006) and the orientation of the suture zone as compared to the kinematic data obtained in this study render the occurrence of a thrusting event improbable, and possibly suggest a transcurrent regime instead.
o Geochronology and in-situ monazite research indicate an alteration of the radiogenic Pb content and instabUity of the monazites probably implying alteration of the granitoids at amphibolite metamorphic conditions.
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