Age of emplacement of massif-type anorthosites in the Eastern Ghats Belt, India: constraints from U–Pb zircon dating and structural studies

Precambrian Research 109 (2001) 25 – 38 www.elsevier.com/locate/precamres

Age of emplacement of massif-type anorthosites in the Eastern Ghats Belt, India: constraints from U–Pb zircon dating and structural studies Olaf Krause a, Christoph Dobmeier b, Michael M. Raith b,*, Klaus Mezger c b

a Verband der Deutschen Feuerfestindustrie, An der Elisabethkirche 27, 53113 Bonn, Germany Mineralogisch-Petrologisches Institut, Uni6ersita¨t Bonn, Poppelsdorfer Schloß, 53115 Bonn, Germany c Institut fu¨r Mineralogie, Uni6ersita¨t Mu¨nster, Corrensstraße 24, 48149 Mu¨nster, Germany

Received 8 February 2000; accepted 19 January 2001

Abstract Massif-type anorthosites in the Proterozoic Eastern Ghats Belt of India occur as complexes of varying size within a strongly deformed, high to ultra-high grade assemblage of metasedimentary and metaigneous rocks. Recently discovered dikes and sheets of exceptionally HFSE and REE-enriched ferrodiorites at the immediate contacts of three anorthosite complexes (Chilka Lake, Bolangir and Turkel) contain ubiquitous magmatic zircon suitable for U – Pb dating. Due to the coeval and comagmatic nature of the ferrodiorites, the U – Pb zircon data provide reliable constraints on the time of anorthosite emplacement. Zircons from three ferrodiorite samples were analysed applying the conventional multi-grain isotope dilution method. Zircon grains in the Bolangir ferrodiorites (D4 and B2012) are long-prismatic and show the faint fine-scale oscillatory zoning typical for magmatic crystallisation, with some poorly luminescent thin overgrowths of metamorphic origin. The data points obtained for abraded grain fractions are slightly discordant and yield an upper intercept age of 933 9 32 Ma with a lower intercept at 515 9 20 Ma, defined by previously published concordant U–Pb titanite data from calc silicate rocks of the border zone of the anorthosite. Zircon grains in the Chilka Lake ferrodiorite (OK 76-2) are prismatic and show zoned igneous interiors that are overgrown by irregular poorly luminescent rims, presumably of metamorphic origin. The U – Pb ages obtained for abraded grain fractions are concordant and define an age of 792 9 2 Ma. The U – Pb zircon ages for the Chilka Lake and Bolangir ferrodiorites disprove the often quoted Mesoproterozoic age of anorthosite emplacement in the EGB, which was based on a four-point Rb–Sr whole rock ‘isochron’ for the Chilka Lake anorthosite complex (1404 9 89 Ma; Sarkar et al., 1981). Combined with field and geochemical evidence, the new U – Pb zircon ages indicate that the data array in the Rb–Sr evolution diagram should be interpreted as a mixing line resulting from marginal contamination of the ascending hot anorthosite pluton with felsic melts generated in its thermal aureole. The U – Pb zircon ages provide new insights in the deformation history of the Eastern Ghats Belt as structural studies indicate intrusion of the Bolangir anorthosite while its country rocks underwent a thrust shear-dominated deformation. In contrast, emplacement of the considerably younger Chilka Lake anorthosite occurred previous to or at the beginning of a regionally intense transpressive deformation. © 2001 Elsevier Science B.V. All rights reserved. * Corresponding author. Fax: + 49-228-732-763. E-mail address: [email protected] (M.M. Raith). 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 1 ) 0 0 1 4 0 - 1

26

O. Krause et al. / Precambrian Research 109 (2001) 25–38

Keywords: U– Pb dating; Zircon; Ferrodiorite; Anorthosite; Eastern Ghats Belt; India

1. Introduction

1.1. Geological background and pre6ious studies Massif-type anorthosites constitute an important component of the Proterozoic Eastern Ghats Belt (Leelanandam, 1987, 1990; Leelanandam and Narashima Reddy, 1988, 1998; Bhattacharya et al., 1998) (Fig. 1). They occur exclusively in the high grade terrane north of the Palaeozoic Godavari rift and form complexes of varying size within a polydeformed association of ultra-high to high grade rocks with sedimentary and magmatic precursors (Sen et al., 1995; Bhattacharya et al., 1998). Previous workers expected the coeval emplacement of all anorthosites, but ideas on the timing of such a major magmatic event with respect to the deformation of the country rocks changed in the course of time. At first, a pre-tectonic emplacement was favoured (De, 1969), but subsequent studies came to the conclusion that the anorthosites intruded during closing stages of deformation (Chilka Lake: Perraju, 1973; Sarkar et al., 1981; Bhattacharya et al., 1994; Sen et al., 1995; Bolangir: Mukherjee et al. 1986; Mukherjee, 1989). Lately, however, an early-tectonic emplacement (=prior to the dominant deformation) has been assumed for the anorthosite complexes at Bolangir and Turkel (Bhattacharya et al. 1995, 1998; Maji et al. 1997). The deformation history of the Eastern Ghats Belt is commonly divided in three individual phases (D1 –D3) that are supposed to have affected the entire orogen (summarised in Bhattacharya, 1997; Mezger and Cosca, 1999). This subdivision is based on an inferred succession of folds with different fold styles and orientations of fold axes. The ‘hook-shaped’ appearance of single layers was attributed to the superimposition of two generations of reclined folds (F1, F2 of respectively D1, D2) with subparallel fold axes of varying orientation. In contrast, generally subhorizontal axes of a third set of upright folds (F3), related to D3, should trend strictly around

W–E. D2 is considered as the principal deformation during which the dominant SW–NE trending foliation formed. Apart from general problems with the conception that an entire orogenic belt was deformed roughly homogeneously (Bhattacharya, 1997), there are few reliable data available to confirm the existence of several independent deformations. New data from the Vishakapatnam-SalurSrikakulam area suggest that the dominant ductile deformation is separated from a relict previous deformation and an ultra-high temperature metamorphic event by an interlude of important crustal magmatism, during which voluminous porphyritic granitoids intruded the khondalitegneiss-pyroxene granulite assemblage at midcrustal levels. This plutonic belt represents one of the world’s largest S-type granitoid provinces (Krause et al., 1996; Mukhopadhyay and Bhattacharya, 1997). Conventional U–Pb zircon dating indicates that the emplacement of porphyritic granitoids took place in the time span between 960 and 985 Ma (Grew and Manton, 1986; Aftalion et al., 1988; Paul et al., 1990; Krause, 1998; Kovach et al., 1998). The magmatic event was followed by a period of intense deformation and subsequent thorough annealing at high temperatures, which has been constrained at 960–935 Ma by conventional U–Pb dating of monazite, allanite and titanite (Aftalion et al., 1988; Paul et al., 1990; Mezger et al., 1996; Mezger and Cosca, 1999) and EPMA dating of monazite (Simmat and Raith, 1998). The isotope data thus provide a narrow time constraint for this deformation, i.e. around 960 Ma. The terrane north of the Godavari Rift also experienced a Pan-African thermal overprint of at least middle-amphibolite facies conditions as indicated by the lower intercept U–Pb ages of discordant titanites and hornblende 40Ar – 39Ar plateau ages (Mezger and Cosca, 1999) as well as EPMA monazite dating (Simmat and Raith, 1998). But so far, no deformation could be attributed to this thermal rejuvenation.

O. Krause et al. / Precambrian Research 109 (2001) 25–38

27

The only published geochronological study on anorthosites in the Eastern Ghats Belt so far is that of Sarkar et al. (1981). These workers attempted to date the Chilka Lake anorthosite using the Rb–Sr method. The isotope data obtained on four anorthositic samples define a linear array which they assigned an isochron age of 14049 89 Ma and interpreted as the formation age of anorthosites. However, there is field and geochemical evidence (Bhattacharya et al., 1995) indicating that the anorthositic sample with the highest Rb/Sr ratio and thus defining the isochron (sample AS/R/24), has been affected by crustal contamination. Anorthositic samples from the marginal zone (including AS/R/24) compared to those from the central portion of the Balugaon massif of the Chilka Lake anorthosite complex are characterised by significantly higher contents of large ion lithophile elements and higher Rb/Sr ratios (Fig. 2). The zonal pattern indicates that contamination of the massif’s margin presumably resulted from interaction of the ascending hot pluton with anatectic melts that formed in its crustal envelope. The linear data array in the Rb –Sr evolution diagram evidently represents a mixing line between two components, the ‘pristine’ anorthosite and a felsic melt component. Fig. 1. Indo-Antarctic terrane assembly in East Gondwana showing the major occurrences of Proterozoic massif-type anorthosite in India and Madagascar (adopted from Sengupta et al., 1999). (1) Chilka Lake (Sarkar et al., 1981), (2) Bolangir (Bhattacharya et al., 1998), (3) Turkel (Maji et al., 1997), (4) Ottanchatram (Janardhan and Wiebe, 1985), (5) Ankafotia (Ashwal et al., 1998), (6) Saririaky (Boulanger, 1959; Ashwal et al., 1998), (7) Manambahy and Volovolo (Boulanger, 1959). India: SC, BC, DC: Archaean Singhbhum, Bastar and Dharwar cratons; N, B, S, MG: granulite terrains of the Nilgiri, Biligirirangan, Shevaroy hill ranges and the Madras region; EGB: Proterozoic Eastern Ghats Belt; MB: Madurai granulite block; KKB: Pan-African Kerala khondalite belt; CB: Cuddapah basin; GR, MR: Godavari and Mahanadi rifts; Madagascar: V, AC: Pan-African Vohibory and Androyan granulite terrains; Sri Lanka: WC, HC, VC: Wanni, Highland and Vijayan complexes; Antarctica: NPC, VHC: granulite terrains of the Archaean Napier and Vestfold Hill complexes; RG, RC, POC, OSG, YBC: Neoproterozoic (with Pan-African overprint) Rauer group, Rayner and Prince Olav complexes, Ongul-Skallen groups and Yamato-Belgica complex; LR, EER: Lambert and East Enderby rifts.

1.2. Scope of this study It is common experience that Rb–Sr and Sm – Nd whole rock methods fail to provide reliable age constraints on massif-type anorthosites, because of the too low variations in the Rb/Sr and Sm/Nd ratios and their sensitivity to crustal contamination (Ashwal, 1993). Minor scale contamination with felsic material, as is the case of anorthosites in the Eastern Ghats Belt (Fig. 2), leads to isotopic ages that are significantly older than the true igneous crystallisation ages. In some cases, well-defined internal Rb–Sr and Sm–Nd mineral-whole rock isochrons have enabled the dating of the metamorphic overprint (Ashwal and Wooden, 1985; Ashwal et al., 1998). Obviously, the best prospect for precise anorthosite chronology offers U–Pb dating of zircon, because of the extremely refractory behaviour of this accessory phase. Unfortunately, the scarcity of zircon in

28

O. Krause et al. / Precambrian Research 109 (2001) 25–38

Fig. 2. Bivariate plots of K2O, Rb, Ba and Rb/Sr vs. MgO to illustrate crustal contamination of the marginal parts of the Chilka Lake anorthosite complex; contaminated and pristine anorthosites are shown by open and filled diamonds, respectively (Bhattacharya et al., 1995), samples studied by Sarkar et al. (1981) by stars.

anorthosites commonly precludes the application of this most promising dating technique. Because of these difficulties, until now the most reliable constraints on the age of massif-type anorthosites have been obtained from U– Pb zircon ages of spatially associated coeval granitoid suites (e.g. McLelland and Chiarenzelli, 1990). Massif-type anorthosites in the Eastern Ghats Belt, as elsewhere in the world, are spatially associated with silicic plutonic rocks ranging in composition from charnockite to ferromonzonite that are interpreted as coeval crustal melts (Bhattacharya et al., 1995, 1998; Maji et al., 1997). At the contacts of the three largest anorthosite complexes (Chilka Lake, Bolangir and Turkel) with border-facies granitoids and supracrustal highgrade gneisses, suites of unique ferrodioritic rocks have been recently discovered (Bhattacharya et al., 1995; Raith et al., 1997). These strongly Feenriched rocks commonly occur at the immediate contact with the anorthosite which they intrude in cross-cutting dikes and sheets. The field and petrographic evidence suggests emplacement of anorthosite closely followed by ferrodioritic rocks, prior to or at the beginning of (Chilka Lake; Dobmeier and Simmat, 2001) or during

(Bolangir; Dobmeier, 2001) a ductile regional deformation and granulite facies recrystallization of the igneous assemblages at mid-crustal levels (750–800°C, 7 –6 kbar). Bhattacharya et al. (1998) interpret the ferrodiorites as interstitial residual melts which, after extraction and segregation from the ascending plagioclase-rich diapirs, experienced selective contamination through interaction with adjacent felsic melts. These processes resulted in exceptionally high concentrations of HFSE and REE (Zr: 5900–1300 ppm, Y: 290 –80 ppm, La: 590–100 ppm, Th: 195–65 ppm, P2O5: 1.7–0.5 wt.% and TiO2: 4.8 – 1.0 wt.%) that are reflected in high modal contents of zircon, thorite (metamict), apatite and ilmenite. Due to the modal abundance of igneous zircon, these rocks offer the opportunity to obtain reliable intrusion ages for the ferrodiorite suite and consequently of the anorthosite emplacement. In this contribution we report and discuss the results of U–Pb zircon dating on ferrodiorites from the anorthosite complexes at Bolangir and Chilka Lake. In order to place the new age data into the context of the geological history of the Eastern Ghats Belt, we present a brief summary of the results of our own structural studies of the Bolangir and Chilka Lake anorthosite complexes and their country rocks with special reference to the structural position of the ferrodiorites. Detailed descriptions of the structural evolution of the two crustal domains are given in Dobmeier and Simmat (2001) and Dobmeier (2001).

2. Results

2.1. Field setting and timing of the emplacement of the ferrodiorites 2.1.1. Bolangir Complexity and special features of the structural pattern as well as the emplacement of pluton-related melts in syn-shortening ferrodiorite and monzonite dikes in and outside the anorthosite imply that the anorthosite complex was emplaced during the progressive compressive deformation of the country rocks (Table 1; Dobmeier, 2001).

O. Krause et al. / Precambrian Research 109 (2001) 25–38

The bulk of the ferrodioritic rocks, which are considered to represent late-stage residual melts of the pluton (Bhattacharya et al., 1998), is concentrated at the contact of the anorthositic rocks with the enclosing garnetiferous granite (Fig. 3a). A minor amount occurs within anorthositic rocks and forms cm to several m wide dikes. The fine-grained (average grain size 0.3 mm) ferrodiorite dikes within the massif trend dominantly NNE–SSW or NW – SE, subparallel to oblique normal shear zones. Some dikes split into a network of variably oriented smaller dikes that reunite, other dikes show high angle offsets and short apophyses or split ends. From these observations it is concluded that angular anorthosite fragments enclosed in ferrodiorite most probably are not xenoliths but in-situ blocks that became isolated in the ferrodiorite during dike propagation. A weak foliation defined by aligned opaque and mafic minerals trends subparallel to the W– E trending post-magmatic foliation in the anorthositic host rock. The ferrodiorite dikes are usu-

29

ally offset at oblique normal shear zones, but may also transect the shear zones within the same locality. These alternate overprinting relationships argue for the emplacement of the dikes during development of the oblique normal shear zones (Table 1). A similar relationship is observed close to the margin. There, the marginal postmagmatic foliation is either transected by the ferrodiorite dikes or it defines the axial planar foliation of isoclinally folded dikes. This implies the concurrence of dike formation and emplacement-related deformation at the anorthosite margin. Ferrodiorites at the anorthosite margin exhibit a planar fabric of considerably varying appearance that contours the contact. In the two roughly wedge-shaped occurrences, which extend for several kilometres into the anorthosite, the pronounced preferred orientation of plagioclase porphyroclasts and an anastomosing foliation marked by mafic minerals define a LS fabric (Bhattacharya et al., 1998; own observations). Locally, drawn out and recrystallised plagioclase

Table 1 Structural evolution of the Bolangir areaa Country rocks

Garnetiferous granite

Primary igneous structures (not related to the regional deformation) – –

Anorthosite+ferrodiorite

Igneous layering; some syndepositional disruption or pre-consolidation deformation indicated

Structures related to the latest Mesoproterozoic (960–945 Ma) progressive regional deformation (D1–D4) D1 Compositional layering (SCL) and – – migmatitic foliation (S1); extensive leucosome formation D2–D3 Repeated isoclinal to open folding Generation/emplacement; composite Emplacement of anorthosite; postmagduring N-directed thrusting accompa- S1–2, contours the contact with the matic foliation trending W–E in the nied by W–E extension; pervasive anorthosite (emplacement-related interior (regional strain) but parallel foliation trends W–E (regional strain) strain); late emplacement of monto the contact near the margin (embut contact-parallel near the garnetif- zonitic dikes which propagate into placement-related strain); NNE–SSW/ erous granite (emplacement-related anorthosite; mixing of granitic and NW–SE trending oblique normal strain); formation of foliation triple ferrodioritic magmas shear zones; emplacement of ferrodiorpoints ite at margin and in anorthosite Late D3 Folding of interfaces between country rocks and garnetiferous granite, respectively, garnetiferous granite and anorthosite massif; emplacement of ferrodiorite in fold hinges at the anorthosite margin and in dilational sites in anorthosite (Fig. 3a) D4 W–E trending weak foliation, locally melt-filled – a Data and interpretation from Dobmeier (2001). Age constraints on the regional deformation are based on geochronological data of Aftalion et al. (1988), Paul et al. (1990), Mezger and Cosca (1999).

30 O. Krause et al. / Precambrian Research 109 (2001) 25–38 Fig. 3. Simplified maps illustrating the geological setting and structural features of massif-type anorthosites at (a) Bolangir (Bhattacharya et al., 1998; Dobmeier, 2001) and (b) Chilka Lake (Dobmeier and Simmat, 2001). The geology is largely based on Bhattacharya et al. (1998) and Sarkar et al. (1981). Due to poor exposure, the country rocks are largely undifferentiated. Stars indicate the ferrodiorite sample locations.

O. Krause et al. / Precambrian Research 109 (2001) 25–38

porphyroclasts constitute rootless isoclinal folds. At the immediate contact, however, formation of a transpositional layering was accompanied by considerable grain size reduction of plagioclase (average grain size 0.2 mm) and isoclinal folding of garnet layers. Obviously, ferrodiorite within the wedges accumulated considerably smaller amounts of strain than in the small sheets, that are intercalated between anorthosite and the garnetiferous granite. This relationship is most likely a consequence of their protected position within the rigid anorthosite. In contrast to the sharp contact with anorthosite, ferrodiorites typically grade into garnetiferous granite, which presumably formed by advective heating and partial melting of country rocks during the emplacement of the anorthosite (Bhattacharya et al., 1998). The observed mixing of ferrodioritic and granitic magmas (Bhattacharya et al., 1998) attests to synchronism of their emplacement and formation. This interpretation is confirmed by the intercalation of garnetiferous granite and ferrodiorite sheets at the immediate contact with anorthosite. Generally, anorthosite is only intercalated with ferrodiorite but never with garnetiferous granite. With the exception of monzonite dikes, which transect the interface at a high angle, granitic rocks do not occur within anorthosite. In contrast, the southern wedge-shaped ferrodiorite occurrence contains several lamellar anorthosite screens and is accompanied by numerous decimetre to metre-wide ferrodiorite dikes. These fine-grained dikes strongly resemble the ferrodiorite dikes in the interior of the anorthosite complex. Anorthosite screens and

31

ferrodiorite dikes trend parallel to the long axis of the ferrodiorite wedge, which is in turn parallel to oblique normal shear zones in the enclosing anorthosite. A fault trending parallel to these shear zones most probably forms the eastern contact (Fig. 3a). Likewise, the northern ferrodiorite wedge trends parallel to the preferred orientation of oblique normal shear zones in the northern part of the anorthosite complex. Taking into account that dike emplacement and shear zone activity coincided in the massif’s interior, both wedges are explained as fillings of tectonic voids that opened in response to the regional deformation. As the northern wedge transects the axial trace of a prominent third fold at right angles (tensile fracture?) but other ferrodiorite occurrences are situated in hinges of third folds, it is concluded that the opening of these voids and the emplacement of the ferrodiorites occurred late (D3) during the regional progressive deformation (Table 1).

2.1.2. Chilka Lake Structural and geochronologic studies from the Chilka Lake area (Dobmeier and Raith, 2000; Dobmeier and Simmat, 2001) document a polyphase and complex tectono-thermal evolution (Table 2). The fabric-defining tectonometamorphic event occurred after the emplacement of the Chilka Lake anorthosite complex. The fine-grained (average grain size 1– 2 mm) and massif ferrodiorites of the Chilka Lake anorthosite complex occur within the anorthosite or at the contact with the country rocks (Fig. 3b).

Table 2 Tectono-magmatic evolution of the Chilka Lake areaa Event

Age First deformation (D1) at ultra-high to high grade metamorphic conditions affected only enderbites and supracrustal rocks; preserved only in low-strain domains Intrusion of leucogranites Emplacement of Chilka Lake anorthosite complex; igneous layering Progressive transpressive deformation (D2–D4) at granulite facies conditions. Formation of a postmagmatic foliation in the anorthosite parallel to S2–3 in the country rocks; final displacement of the anorthosite along a NNE–SSW dextral shear zone? Thermal activity

1 2 3 4

5 a

Data and interpretation from Dobmeier and Raith (2000) and Dobmeier and Simmat (2001).

Probably 960 Ma ? 792 92 Ma 690–663 Ma

510 Ma

O. Krause et al. / Precambrian Research 109 (2001) 25–38

32

Table 3 Major and trace element chemistry of ferrodiorite samples (wt.%)

B2012

D4

OK76-2

SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O P2O5

35.12 4.12 7.80 37.65 0.56 3.22 4.93 1.20 0.31 1.55

37.35 4.45 7.75 35.89 0.58 3.63 4.57 1.18 0.44 1.48

50.45 3.16 13.85 20.27 0.38 3.25 6.58 1.15 0.20 0.03

Total

97.08

97.31

99.32

(ppm) Cr Ni Sc V Zn

59 43 51 98 480

60 43 73 131 489

102 49 76 221 197

Rb Ba Sr

4 81 120

16 95 110

2 80 195

Nb Zr Y Th

264 5512 152 87

282 5100 237 183

43 1521 25 2

La Ce

431 829

441 1066

72 49

a

Total iron as FeO.

In the interior of the complex, several metres wide and up to 100 m long ferrodiorite bodies are associated with meta-leuconorite and metanorite of unusual geochemical composition, which indicates crustal contamination (unpublished data). As all primary structures were destroyed during the following regional deformation, the timing of ferrodiorite emplacement in the massif’s interior remains unknown. Likewise, relationships between ferrodiorite and adjoining rocks are hard to establish because the contact is poorly exposed. However, several field observations place constraints on the timing. Close to the contact, lenticular aggregates of bluish quartz occur in anorthosite and ferrodiorite. Considering the chemical composition of the

anorthositic rocks and the presence of garnet therein (unpublished data), the lenticles most probably result from crustal contamination. Further, cm-scale ferrodiorite layers are intercalated with highly restitic quartz–garnet–sillimanite– spinel gneisses which we interpret to be a product of partial melting of khondalitic rocks during ultra-high temperature contact metamorphism. In conclusion, a coeval emplacement of ferrodiorite and anorthosite seems most likely.

2.2. Isotope studies Three ferrodiorite samples were selected for U–Pb zircon dating. Samples D4 and B2012 were collected from ferrodiorite dikes at the southern margin of the Bolangir anorthosite complex (Fig. 3a), sample OK76-2 from a ferrodiorite sheet near the north-eastern contact of the Balugaon massif of the Chilka Lake anorthosite (Fig. 3b). The ferrodiorites from the two complexes are characterised by somewhat different bulk chemistry, which is reflected by the assemblages and mineral compositions. The extremely femic Bolangir ferrodiorites are characterised by the assemblage orthopyroxene+fayalite+clinopyroxene+coronitic garnet+ megacrystic plagioclase+ilmenite. In contrast, the more felsic Chilka Lake ferrodiorite contains no fayalite but quartz may be present. Tables 3 and 4 present the major and trace element compositions, full assemblages, selected mineral composition data and information on textural features of the three samples. For further information on the petrography, geochemistry and petrogenesis of ferrodiorites the reader is referred to Maji et al. (1997), Raith et al. (1997) and Bhattacharya et al. (1998). Zircons were extracted by magnetic and heavy liquid (methylene iodide) separation. Inclusionfree zircons then were hand picked and abraded in two steps (50 and 80% of the whole grain). U–Pb analysis of the zircon fractions was performed at the Max-Planck-Institut fu¨ r Chemie in Mainz, applying the conventional multi-grain isotopic dilution method. For details of the analytical procedures see Seth et al. (1998). All data

Table 4 Mineral composition data, assemblages and textural features of ferrodiorite samplesa D4 grt

OK76 grt

B2012 fay

D4 fay

B2012 opx

D4 opx

OK76 opx

B2012 cpx

D4 cpx

SiO2 TiO2 Al2O3 FeOb MnO MgO CaO Na2O K2O

37.04 0.09 19.90 30.86 1.33 1.81 7.33

37.05 0.06 20.48 31.28 1.36 2.21 7.03

37.46 0.03 20.08 31.60 1.39 1.89 7.62

30.21 0.04 0.00 63.25 0.77 4.80 0.02

29.86 0.02 0.00 63.62 0.73 5.39 0.01

47.45 0.10 0.72 41.45 0.60 8.48 1.06 0.04

47.14 0.16 0.62 40.97 0.75 8.45 1.09 0.00

48.16 0.07 0.39 39.93 0.46 10.33 0.73 0.00

48.90 0.31 1.68 23.51 0.36 6.53 19.07 0.40

48.38 0.28 1.63 21.59 0.30 6.99 19.08 0.36

Total

98.35

99.47

100.10

99.09

99.67

99.91

99.22

100.10

100.80

98.60

Oxygens

24

24

24

Si Ti Al Fe Mn Mg Ca Na K XFe

4

4

6

6

6

6

OK76 cpx

B2012 am

D4 am

50.53 0.14 1.03 19.34 0.27 8.55 20.50 0.23

38.94 3.15 11.20 24.76 0.15 4.77 10.66 1.87 2.13

38.71 2.31 11.78 23.96 0.15 5.52 11.06 1.78 2.29

60.90 0.06 24.81 0.11 0.00 0.00 6.42 7.94 0.49

59.79 0.00 25.24 0.09 0.03 0.00 7.20 7.34 0.62

49.50 0.02 31.96 0.30 0.04 0.00 13.97 3.33 0.15

100.60

97.65

97.59

100.73

100.37

99.25

23

23

8

8

8

2.70 0.00 1.29 0.00 0.00 0.00 0.30 0.68 0.03

2.66 0.00 1.33 0.00 0.00 0.00 0.34 0.63 0.04

2.27 0.00 1.73 0.00 0.00 0.00 0.69 0.30 0.01

6

6

6.04 0.01 3.83 4.21 0.18 0.44 1.28

5.99 0.01 3.90 4.22 0.19 0.53 1.22

6.01 0.01 3.79 4.24 0.19 0.45 1.31

1.00 0.00 0.00 1.75 0.02 0.24 0.00

0.98 0.00 0.00 1.75 0.02 0.26 0.00

1.96 0.00 0.04 1.43 0.02 0.52 0.05 0.00

1.96 0.01 0.03 1.42 0.03 0.52 0.05 0.00

1.95 0.00 0.02 1.35 0.02 0.62 0.03 0.00

1.93 0.01 0.08 0.78 0.01 0.39 0.81 0.03

1.94 0.01 0.08 0.72 0.01 0.42 0.82 0.03

1.96 0.00 0.05 0.63 0.01 0.49 0.85 0.02

0.91

0.89

0.90

0.88

0.87

0.74

0.73

0.67

0.67

0.63

0.54

6.18 0.38 2.09 3.29 0.02 1.13 1.81 0.57 0.43 0.75

6.13 2.20 0.28 3.17 0.02 1.30 1.88 0.55 0.46 0.71

B2012 plg

D4 plg

Ok76 plg O. Krause et al. / Precambrian Research 109 (2001) 25–38

B2012 grt

a B2012, fayalite+orthopyroxene+clinopyroxene+plagioclase+garnet+ferropargasite+ilmenite+po, ccp, ap, zrn, thorite; porphyritic, foliated, granoblastic; D4, fayalite+orthopyroxene+clinopyroxene+plagioclase+garnet+ferropargasite+ilmenite+po, ccp, ap, zrn, thorite; porphyritic, massive, granoblastic; OK76, orthopyroxene+clinopyroxene+garnet+plagioclase+quartz+ilmenite+po, pn, ccp, ap, zrn; porphyritic, foliated, granoblastic. b Total iron as FeO.

33

O. Krause et al. / Precambrian Research 109 (2001) 25–38

34

Table 5 U–Pb data for zircon from ferrodiorites of the Bolangir and Chilka Lake anorthosite complexes Sample

U Pb Ratio (ppm) (ppm) Pb/204Pba

Age (Ma)

206

208

Pb/206Pbb

207

Pb/206Pbb

207

Pb/235Ub

206

Pb/238Ub

206

Pb/238U

207

Pb/235U

207

Pb/206Pb

Bolangir B2012 B2012 ab1 B2012 ab2

298 207 45

45 36 7

1118 417.2 609.2

0.2800 0.3010 0.3136

0.06655(10) 1.170(5) 0.06776(41) 1.269(9) 0.06777(30) 1.235(7)

0.1275(6) 0.1359(6) 0.1321(5)

773 821 800

786 832 786

824 861 862

D4 D4 ab1

374 240

54 37

2828 2521

0.2580 0.2535

0.06692(4) 1.153(3) 0.06776(11) 1.236(4)

0.1249(5) 0.1323(6)

759 801

778 817

835 861

Chilka Lake OK 76-2 490 OK76-2 ab1 486 OK76-2 ab2 8

78 78 8

5206 5272 618

0.3997 0.3494 0.4551

0.06596(4) 0.06552(5) 0.06559(7)

0.1253(5) 0.1302(5) 0.1304(5)

761 789 790

772 790 791

805 791 793

a b

1.139(3) 1.176(3) 1.179(3)

Measured ratio. Corrected for fractionation, blank and common Pb; ab1 = 50% abraded, ab2 =80% abraded.

were reduced using a modified version of the program Pb-DAT (Ludwig, 1980, 1998). The uncertainties reported for the U– Pb and 207Pb/ 206 Pb ages (2|) include the reproducibility of the standard, common lead and blank corrections as well as the uncertainties in the U/Pb of the spike. The analytical data are presented in Table 5.

2.2.1. Bolangir Zircons of samples D4 and B2012 are longprismatic, colourless clear to yellowish euhedral grains. Cathodoluminescence images reveal a complex internal structure (Fig. 4). Distinct long prismatic cores are overgrown by broad rims that show somewhat irregular fine-scale oscillatory zonation considered typical for magmatic crystallisation. The cores host elongate multiphase inclusions interpreted as former melt inclusions. These features indicate that the cores most likely represent early cumulus crystals that settled in the fractionating ferrodioritic melt rather than crystals that were taken up from adjacent felsic crustal melts (i.e. the garnetifer-

ous granite). The rarely developed poorly luminescent thin rims at the terminations of the grains are presumably of metamorphic origin. The U –Pb data points obtained for nonabraded and abraded grain fractions are discordant (Fig. 5a). In the absence of ion-microprobe data, the interpretation of the discordance is not straightforward. The data array could indicate the presence of inherited older cores mantled by rims that crystallised from the ferrodioritic melt. However, judging from the internal structure of the grains, a cumulate origin and thus an almost synchronous crystallisation of cores and rims appears more likely. We therefore attribute the discordance to partial Pbloss during the Pan-African thermal overprint of the area which is documented by near-concordant U–Pb titanite data for calc silicate rocks of the crustal envelope of the anorthosite (5159 20 Ma; Mezger and Cosca, 1999). The discordia through this Pan-African age yields an upper intercept age of 9339 32 Ma which is interpreted as an upper age limit for ferrodiorite crystallisation.

O. Krause et al. / Precambrian Research 109 (2001) 25–38

35

2.2.2. Chilka Lake Zircons of the Chilka Lake ferrodiorite sample (OK 76-2) are clear, prismatic to short-prismatic multi-faceted euhedral grains. Cathodoluminescence images show zoned igneous cores that are overgrown by irregular poorly luminescent rims, presumably of metamorphic origin (Fig. 4). The data points obtained for abraded grain fractions are concordant and give an age of 79292 Ma which is interpreted as a magmatic crystallisation age of the ferrodiorite (Fig. 5b). Because of the coeval nature of both the ferrodiorite and the anorthosite, the U– Pb zircon date provides a reliable age constraint for the emplacement of the Chilka Lake anorthosite complex. The U– Pb zir-

Fig. 5. Concordia diagrams showing the U – Pb zircon data for ferrodiorite samples from (a) the Bolangir (D4 and B2012) and (b) the Chilka Lake anorthosite complexes. The isotope data are given in Table 5.

con age, in addition to the chemical arguments presented in the introduction, disproves the interpretation of the Rb–Sr whole rock data of Sarkar et al. (1981) for the Chilka Lake anorthosite as an isochron-age and consequently the often quoted mid-Proterozoic age of anorthosites in the Eastern Ghats Belt (e.g. Ramakrishnan et al., 1998).

3. Conclusions Fig. 4. Cathodoluminescence images of sections through zircon grains from ferrodiorite samples of the Bolangir (D4 and B2012) and Chilka Lake (OK 76-2) anorthosite complexes.

The new U –Pb zircon ages for ferrodiorites associated with the Chilka Lake and Bolangir anorthosite complexes imply at least two episodes

36

O. Krause et al. / Precambrian Research 109 (2001) 25–38

of anorthosite intrusion. Combined with new constraints on the emplacement timing in relation to the regional deformation, the U– Pb zircon data provide important new insights into the history of the Eastern Ghats Belt. The age for intrusion of the Bolangir anorthosite overlaps with the time of the main regional tectono-metamorphic episode that affected most of the orogen. This age of 933 Ma with a large uncertainty of 32 Ma does not allow a correlation of the anorthosite emplacement with a specific segment of the P-T evolution. The structural studies, on the other hand, show that the anorthosite complex was emplaced during the regional deformation. Accordingly, the ages of 965– 935 Ma from metamorphic monazite and allanite (Aftalion et al., 1988; Paul et al., 1990; Mezger et al., 1996; Mezger and Cosca, 1999) and EPMA monazite dating (Simmat and Raith, 1998), constraining the time of Grenvillian peak metamorphism and early cooling in the Eastern Ghats Belt, set a minimum age for the intrusion of the Bolangir anorthosite. The concordant age of 79292 Ma for the Chilka Lake ferrodiorite proves that emplacement of the anorthosite evidently post-dates the Grenvillian deformation and high grade metamorphism. As a consequence, the episode of ductile transpressive deformation and high grade metamorphism that has affected the Chilka Lake anorthosite belongs to a separate orogenic event. Recent EPMA dating of monazite from different lithologies of the Chilka Lake area corroborates this conclusion (Simmat and Raith, 1998; Dobmeier and Simmat, 2001). The regional extent of this deformation is not established beyond the coastal parts of the northernmost Eastern Ghats Belt. The combination of isotope data with structural studies has been proven to be a powerful tool for the reconstruction of a significant part of the geodynamic evolution of this Proterozoic orogenic belt. The anorthosite complexes in the Eastern Ghats Belt represent geochronological field markers that allow to constrain the timing of the fabric-defining regional deformation in the neighbouring gneisses. As shown in this study, there were at least two discrete events that led to the emplacement of anorthosites. In consequence, chemical similarity cannot be used to infer coeval

genesis. The results further demonstrate that the Eastern Ghats Belt comprises crustal domains that were subjected to different deformation regimes at appreciably different times.

Acknowledgements This study received benefit from collaborative research with A. Bhattacharya (IIT, Kharagpur), S. Dasgupta and P. Sengupta (Jadavpur University, Calcutta), J.K. Nanda (GSI, Bhubaneswar), S. Hoernes and R. Simmat (Bonn). We are grateful to these colleagues for their helpful contributions. S. Klerner (Ko¨ ln) kindly provided cathodoluminescence images of zircons from the analysed samples. Field studies and analytical work were supported by the Deutsche Forschungsgemeinschaft (grants Ra 205/20 and Do 644/1). Constructive reviews by D.R. Nelson and M. Yoshida helped to improve the paper. This is a contribution to IGCP-368 ‘Proterozoic Events in East Gondwana’.

References Aftalion, M., Bowes, D.R., Dempster, T.J., 1988. Late Proterozoic charnockites in Orissa, India: a U – Pb and Rb– Sr isotopic study. J. Geol. 96, 663 – 676. Ashwal, L.D., 1993. Anorthosites. Minerals and Rocks 21. Springer-Verlag, Berlin, p. 422. Ashwal, L.D., Wooden, J.L., 1985. Sm-Nd isotopic studies of Proterozoic anorthosites: systematics and implications. In: Tobi, A.C., Touret, J.L.R. (Eds.), The Deep Proterozoic Crust in the North Atlantic Provinces, NATO ASI Series C, vol. 158. Reidel, Dordrecht, pp. 61 – 73. Ashwal, L.D., Hamilton, M.A., Morel, V.P.I., Rambelson, R.A., 1998. Geology, petrology and isotope geochemistry of massif-type anorthosites from southwest Madagascar. Contrib. Mineral. Petrol. 133, 389 – 401. Bhattacharya, S., 1997. Evolution of the Eastern Ghats granulite belt of India in a compressional tectonic regime and juxtaposition against Iron Ore Craton of Singhbhum by oblique collision-transpression. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 106, 65 – 75. Bhattacharya, A., Raith, M., Hoernes, S., 1995. The anorthosite-ferrodiorite-monzonite suite at Bolangir, Eastern Ghats Belt, India. Terra Nova 7 (Abstract Supplement 1), 308. Bhattacharya, A., Raith, M., Hoernes, S., Banerjee, D., 1998. Geochemical evolution of the massif-type anorthosite com-

O. Krause et al. / Precambrian Research 109 (2001) 25–38 plex at Bolangir in the Eastern Ghats Belt of India. J. Petrol. 39, 1169 – 1195. Bhattacharya, S., Sen, S.K., Acharyya, A., 1994. The structural setting of the Chilka Lake granulite-migmatiteanorthosite suite with emphasis on the time relation of charnockites. Precambrian Res. 66, 393 –409. Boulanger, J., 1959. Les anorthosites de Madagascar. Annal. Ge´ ol. Madagascar 26, 1 –71. De, A., 1969. Anorthosites of the Eastern Ghats, India. In: Isachsen, Y.W. (Ed.), Origin of Anorthosite and Related Rocks. NY State Mus. Sci. Ser. Mem, vol. 18, pp. 425 – 434. Dobmeier, C., 2001. The Bolangir massif-type anorthosite complex (Eastern Ghats Belt, India): evidence for anorthosite emplacement during crustal thickening. Canadian Journal of Earth Sciences (submitted). Dobmeier, C., Raith, M., 2000. On the origin of ‘arrested’ charnockitization in the Chilka Lake area, Eastern Ghats Belt, India — a reappraisal. Geol. Mag. 137, 27 –37. Dobmeier, C., Simmat, R., 2001. Post-Grenvillian transpression in the Chilka Lake area, Eastern Ghats Belt-implications for the amalgamation history of peninsular India. Precambrian Res. (in revision). Grew, E.S., Manton, W.I., 1986. A new correlation of sapphirine granulites in the Indo-Antarctic metamorphic terrain: Late Proterozoic dates from the Eastern Ghats. Precambrian Res. 33, 123 –139. Janardhan, A.S., Wiebe, R.A., 1985. Petrology and geochemistry of the Oddanchatram anorthosite and associated basic granulites, Tamil Nadu, South India. J. Geol. Soc. India 26, 163 – 176. Kovach, V.P., Berezhnaya, N.G., Salnikova, E.B., Narayana, B.L., Divakara Rao, V., Yoshida, M., 1998. U –Pb zircon age and Nd isotope systematics of megacrystic charnockites in Eastern Ghats granulite belt, India, and their implication for East Gondwana reconstruction. J. African Earth Sci. 27, 125 – 127 (Special Abstract Issue ‘Gondwana-10: Event Stratigraphy of Gondwana’). Krause, O., 1998. Die petrogenetische Bedeutung der porphyrischen Granitoide fu¨ r die Krustenentwicklung des Eastern Ghats Belt (Indien). Ph.D. Thesis, University of Bonn, Germany. Cuvillier Verlag, Go¨ ttingen, p. 203. Krause, O., Mezger, K., Raith, M., Bhattacharya, A., 1996. First petrogenetical conclusions on megacrystic S-type granitoids from the Eastern Ghats Belt (India). J. Conference Abstr. 1, 334. Leelanandam, C., 1987. Proterozoic anorthosite massifs: an overview. Indian J. Geol. 59, 179 –194. Leelanandam, C., 1990. The anorthosite complexes and Proterozoic mobile belt of Peninsular India. In: Naqvi, S.M. (Ed.), Precambrian Continental Crust and Its Economic Resources. Developments in Precambrian Geology 8. Elsevier, Amsterdam, pp. 409 –435. Leelanandam, C., Narashima Reddy, M., 1988. Precambrian anorthosites from Peninsular India-problems and perspectives. Indian J. Geol. 60, 111 –136.

37

Leelanandam, C., Narashima Reddy, M., 1998. Precambrian anorthosites from Peninsular India-problems and perspectives. Geol. Surv. Ind. Spec. Pub. 44, 152 – 169. Ludwig, K.R., 1980. Calculation of uncertainties of U – Pb isotope data. Earth Planet. Sci. Lett. 46, 212 – 220. Ludwig, K.R., 1998. Using Isoplot/Ex: A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication No. 1, p. 42. Maji, A.K., Bhattacharya, A., Raith, M., 1997. The Turkel anorthosite complex revisited. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 106, 313 – 325. McLelland, J.M., Chiarenzelli, J., 1990. Isotopic constraints on emplacement age of anorthositic rocks of the Marcy massif, Adirondack Mts., New York. J. Geol. 98, 19 – 41. Mezger, K., Cosca, M.A., 1999. The thermal history of the Eastern Ghats Belt (India) as revealed by U – Pb and 40 Ar/39Ar dating of metamorphic and magmatic minerals: implications for the SWEAT correlation. Precambrian Res. 94, 251 – 271. Mezger, K., Cosca, M.A., Raith, M., 1996. The thermal history of the Eastern Ghats Belt (India) deduced from U – Pb and 40Ar– 39Ar dating of metamorphic minerals. J. Conference Abstr. 1, 401 – 402. Mukherjee, A., 1989. P-T-time history and thermal modelling of an anorthosite-granulite interface, Eastern Ghats metamorphic belt, India. In: Daly, J.S., Cliff, R.A., Yardley, B.W. (Eds.), Evolution of Metamorphic Belts. Geol. Soc. Spec. Publ, vol. 43, pp. 265 – 274. Mukherjee, A., Bhattacharya, A., Chakraborty, S.C., 1986. Convergent phase equilibria at the massif anorthosite-granulite interface near Bolangir, Orissa, India, and the thermal evolution of a part of the Indian Shield. Precambrian Res. 34, 69 – 104. Mukhopadhyay, A.K., Bhattacharya, A., 1997. Tectonothermal evolution of the gneiss complex at Salur in the Eastern Ghats belt of India. J. Metamorphic Geol. 15, 719 – 734. Paul, D.K., Ray Barman, T., McNaughton, N.J., Fletcher, I.R., Potts, P.J., Ramakrishnan, M., Augustine, P.F., 1990. Archean-Proterozoic evolution of Indian charnockites: isotopic and geochemical evidence from granulites of the Eastern Ghats Belt. J. Geol. 98, 253 –263. Perraju, P., 1973. Anorthosite of Puri district, Orissa. Ind. Minerals 14, 310. Raith, M., Bhattacharya, A., Hoernes, S., 1997. A HFSE- and REE-enriched ferrodiorite suite from the Bolangir anorthosite complex, Eastern Ghats Belt, India. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 106, 299 – 311. Ramakrishnan, M., Nanda, J.K., Augustine, P.F., 1998. Geological evolution of the Proterozoic Eastern Ghats mobile belt. Geol. Surv. India Spec. Pub. 44, 1 – 21. Sarkar, A., Bhanumathi, L., Balasubrahmanyan, M.N., 1981. Petrology, geochemistry, and geochronology of the Chilka Lake igneous complex, Orissa State, India. Lithos 14, 93 – 111. Sen, S.K., Bhattacharya, S., Acharyya, A., 1995. A multi-stage pressure-temperature record in the Chilka Lake granulites: the epitome of the metamorphic evolution of Eastern Ghats, India? J. Metamorphic Geol. 13, 287 – 298.

38

O. Krause et al. / Precambrian Research 109 (2001) 25–38

Sengupta, P., Dasgupta, S., Raith, M.M., Bhui, U., Ehl, J., 1999. Ultra-high temperature metamorphism of metapelitic granulites from Kondapalle, Eastern Ghats Belt: Implications for the Indo-Antarctic correlation. J. Petrol. 40, 1065 – 1087. Seth, B., Kro¨ ner, A., Mezger, K., Nemchin, A.A., Pidgeon, R.T., Okrusch, M., 1998. Archaean to Neopro-

terozoic magmatic events in the Kaoko belt of NW Namibia and their geodynamic significance. Precambrian Res. 92, 341 – 363. Simmat, R., Raith, M., 1998. EPMA monazite dating of metamorphic events in the Eastern Ghats Belt of India. Beih. 1 z. Eur. J. Mineral. 10, 276.

.