华夏地块显生宙的变质作用期次和特征

华夏地块主要存在四期变质作用。加里东期变质作用呈北东向展布于华夏的大部分地区,变质作用可达麻粒岩相,且麻粒岩断续分布平行于造山带,此期变质作用是在挤压造山构造背景下发生,很可能与扬子地块向冈瓦那大陆北缘聚合–碰撞,造成大陆边缘沉积物变形–变质有关。根据粤东梅县片麻岩和兴宁混合岩的LA–ICPMS锆石U–Pb定年以及邻区独居石U–Pb年代学的研究,海西的变质作用主要发生在260~280 Ma,年轻于欧洲典型的海西期造山时代。华夏地块的海西期变质作用分布局限,它们可能形成于拉张构造背景。印支期变质岩在华夏有较广泛的分布,西南端大容山—十万大山的印支期变质作用可达麻粒岩相,其他地区的变质作用具有中低压相系的特征,记录了造山后期伸展构造背景。LA–ICPMS锆石U–Pb定年指示华夏中部粤中地区的印支期变质作用发生在231~232 Ma。燕山期变质岩主要分布于东南沿海和台湾中央山脉,显示了双变质带的特点,表明与太平洋板块向东南沿海俯冲作用密切相关。从印支期到燕山期,变质带的方向发生了转变,说明影响华夏地块变质作用的构造域发生了改变。     

Metamorphic Evolution of High‐pressure Politic Granulite from the Ailaoshan Orogenic Belt,Northeastern Margin of Indochina Block,West Yunnan,China

Ailaoshan orogenic belt located at the northeastern margin of the Indochina block, southeastern Tibet, was formed by subduction and collision between the Indochina and South China blocks in Triassic and slip shearing resulted from the extrusion of the Indochina block in Cenozoic. The high‐pressure pelitic granulite is located at the southeastern margin of the Ailaoshan metamorphic belt, occurs as a slice of about 500～700m in thickness, consists of garnet, sillimanite, feldspar, biotite and quartz with accessory of kyanite, sapphirine, spinel, rutile, ilmenite, zircon and apatite. The petrography and mineral chemistry show that the high‐pressure pelitic granulite had suffered three stages of metamorphism: 1) the prograde metamorphism recorded by the mineral assemblage of garnet, kyanite, feldspar, biotite and rutile; 2) the peak metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, ternary feldspar, K‐feldspar, plagioclase, biotite, spinel, quartz, rutile and zircon mantle; 3) the retrograde metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and zircon rim. Zircon SHRIMP U‐Pb dating indicates that the protolith of the pelite granulite was deposited before 336 Ma, the prograde to peak metamorphism occurred at P‐T conditions of ≥10.4 kbar at 850～919 °C in 235 Ma, and the retrograde metamorphism occurred at the P‐T condition of 3.5～3.9 kbar at 572～576 °C until to 33 Ma. They are consistent with the times of Indochina separated from Gondwanaland during late Paleozoic, the amalgamation of the south China and Indochina blocks during the Triassic, and the sinistral slip‐shearing since the Early Cenozoic respectively. It is inferred that that the sedimentary rock was subducted to the lower continental crust (30 km) and suffered granulite‐facies metamorphism due to the collision during Indosinian, then exhumed quickly to middle‐upper crust (10–12km) and superimposed retrograde metamorphism since the Cenozoic.     

Contrasting magmatic signatures in the Rairakhol and Koraput alkaline complexes,Eastern Ghats belt,India

The relation between alkaline magmatism and tectonism has been a contentious issue, particularly for the Precambrian continental regions. Alkaline complexes at the southwestern margin of Eastern Ghats belt, India, have been interpreted as rift-valley magmatism. However, those complexes occurring in granulite ensemble in the interior segments of the Eastern Ghats belt could not possibly be related to the rift-system, assumed for the western margin of the Eastern Ghats belt. Koraput complex was emplaced in a pull-apart structure, dominated by magmatic fabrics and geochemically similar to a fractionated alkaline complex, compatible with an alkalibasalt series. Rairakhol complex, on the other hand, shows dominantly solid-state deformation fabrics and geochemically similar to a fractionated calc-alkaline suite. Isotopic data for the Koraput complex indicate ca. 917 Ma alkaline magmatism from a depleted mantle source and postcrystalline thermal overprint at ca. 745 Ma, also recorded from sheared metapelitic country rocks. The calc-alkaline magmatism of the Rairakhol complex occurred around 938 Ma, from an enriched mantle source, closely following Grenvillian granulite facies imprint in the charnockitic country rocks.     

Single zircon ages from high-grade rocks of the Jianping Complex, Liaoning Province, NE China

The high-grade rocks of the Jianping Complex in Liaoning Provi nce, NE China, belong to the late Archaean to earliest Proterozoic granulite belt of the North China craton. Single zircon ages obtained by the Pb–Pb evaporation method and SHRIMP analyses document an evolutionary history that began with deposition of a cratonic supracrustal sequence some 2522–2551 Ma ago, followed by intrusion of granitoid rocks beginning at 2522 Ma and reaching a peak at about 2500 Ma. This was followed by high-grade metamorphism, transforming the existing rocks into granulites, charnockites and enderbites some 2485–2490 Ma ago. The intrusion of post-tectonic granites at 2472 Ma is associated with widespread metamorphic retrogression and ends the tectono–metamorphic evolution of this terrain. A similar evolutionary sequence has also been recorded in the granulite belt of Eastern Hebei Province. We speculate that the Jianping Complex was part of an active continental margin in the late Archaean that became involved in continental collision and crustal thickening shortly after its formation. There is a remarkable similarity between the 2500 Ma North China granulite belt and the equally old granulite belt of Southern India, suggesting that the two crustal domains could have been part of the same active plate margin in latest Archaean times.     

一条早前寒武纪的陆-陆碰撞造山带——华北克拉通北缘中西部麻粒岩相带

详细的野外调查、原岩建造、变质作用、变形作用以及同位素年代学研究表明,华北克拉通北缘中西部麻粒岩相带是我国迄今能识别的最老的陆-陆碰撞造山带的根带,可与世界上著名的南非Limpopo碰撞造山带对比。     

Contrasting metamorphism across cauvery shear zone, south India

The Palghat Cauvery Shear Zone (CSZ) is a major shear zone that possibly extends into different fragments of Gondwanaland. In the present study mafic granulites occurring on either side of the CSZ in Namakkal area, southern India are examined. Textural features recorded in the mafic granulites are crucial in elucidating the metamorphic history of the southern granulite terrane (SGT). In the mafic granulites occurring to the south of CSZ, evidence of garnet breaking down during near isothermal decompression (ITD) is indicated by the development of orthopyroxene + plagioclase moats in between quartz and garnet. The presence of comparatively small elongated second generation garnet embedded in pyroxenes from the mafic granulites occurring to the north of CSZ is indicative of the garnet formation via reaction between pyroxenes and plagioclase, which occurred during isobaric cooling (IBC). Rocks occurring to the south of CSZ have recorded comparatively higher temperature and pressure (849‡C and 9.6kbar) than those occurring to the north of the CSZ (731‡C and 8.6kbar) using conventional geothermobarometry. The rocks occurring to the north of CSZ have suffered more complex metamorphic histories in comparison to the southern part. Integrating the results of the present field and metamorphic studies with the earlier investigations and available geochronological data we suggest that the CSZ could represent a suture zone between two different continental blocks that underwent distinct metamorphic evolution.     

Evolution of the Western Margin of Australia during the Rodinian and Gondwanan Supercontinent Cycles

The proto-Darling Fault zone and its successor, the Darling Fault, extend for 1, 000 km along the western continental margin of Australia and appear to have been active at several periods during the geological past. Deformation commenced at 2,570 Ma and affected Late Archaean granitoids along the western margin of the Yilgarn Craton. Much of the later activity reflects events related to the accretion and breakup associated with the Rodinia and Gondwanaland supercontinent cycles.In the north, rocks of the Northampton and Mullingarra Complexes form part of a high-grade Grenvillian orogenic belt lying to the west of the Darling Fault, referred to as the Pinjarra Orogen. They underwent granulite facies metamorphism 1080 Ma ago and form part of the global collisional event that resulted in the amalgamation of Rodinia. These rocks extend southward beneath Phanerozoic sedimentary cover (the Perth Basin), where they are constrained to the east by the Darling Fault and to the west by the Dunsborough Fault, the latter marking the eastern boundary of the Leeuwin Complex.The Leeuwin Complex is a fragment of Pan-African crust that has traditionally been considered part of the Pinjarra Orogen. It is composed predominantly of upper amphibolite to granulite facies felsic orthogneisses derived from A-type, anorogenic granitoids. Conventional and SHRIMP U-Pb zircon geochronology has established that the granitoids evolved between 780 Ma and 520 Ma and were metamorphosed at 615 Ma. These events are equated with rifting associated with the breakup of Rodinia. Sm-Nd whole rock data support the juvenile nature of the crust and provide no evidence for the involvement of pre-existing Archaean continental material.During the Phanerozoic, the Dunsborough and Darling Faults were reactivated, as normal faults defining the inner arm of a major rift system within Eastern Gondwanaland and controlling sedimentation in the Perth Basin that now overlies the Grenvillian terrane. Major normal movement on the Darling Fault ceased by the Late Jurassic and it appears that continental breakup in the Early Cretaceous occurred along fractures closely related to the western boundary of the Leeuwin Complex that defined the eastern margin of the outer arm of the rift system. Breakup between Australia and Greater India commenced at 132 Ma and was followed by eruption of the Bunbury Basalt at 130 Ma and 123 Ma. This possibly resulted from hot spot activity beneath Eastern Gondwanaland and may have been a reflection of the Kerguelen plume, though the evidence is equivocal.It is argued from the petrographic, geochemical and isotopic characteristics, together with the likely contiguity of the Eastern Gondwanaland continents since the assembly of Rodinia, that the Leeuwin Complex evolved within an intracrustal rift and is not an exotic terrane. It is distinct from adjacent portions of the Pinjarra Orogen and should be considered a separate terrane. It is recommended that use of the term ‘Pinjarra Orogen’ be confined to rocks recording the Grenvillian events, thereby excluding those rocks (the Leeuwin Complex) that evolved during the later Pan-African orogeny.     

Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal

The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six north-south trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100–150 km wide dextral shear zone of probable Pan-African age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic -ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.     

Seismotectonic studies of the coastal areas of India,Pakistan, Bangladesh,and Burma

Major geotectonic elements that are seismically active in the near-shore areas of the Indian subcontinent are the Mekran fault off the coast of Pakistan, the western part of the Narmada-Son lineament, the West Coast Fault off the west coast of India - a southward extension of the Cambay Rift, the Palghat Gap, the Godavari and Mahanadi grabens, transecting rather at an angle to the eastern coast of India and the Arakan-Yoma arcuate belt of Burma, which is a part of the global Alpine-Himalayan orogenic belt, continuing southwards into the Andaman-Nicobar island complex and the Java-Sumatra trench on the ocean floor of the advancing Indo-Australian Plate.The coastal belt exhibits varied degrees of seismicity from intensely seismic areas, like the Mekran coast off Pakistan, Kutch (India) and the Arakan-Yoma belt of Burma, with earthquake magnitudes of more than 8.0, while the intervening coastal areas of the Peninsular India are moderately seismic to aseismic. The remaining areas, namely, the major part of the coastal belt of Bay of Bengal in India and Bangladesh are broadly aseismic. However, the active Godavari graben and the eastern part of the coast of Bangladesh are frequented by low to moderate magnitude earthquakes. An extension of the active Arakan-Yoma belt in the Bay of Bengal in the form of the Andaman-Nicobar Island complex is highly seismic with a maximum earthquake magnitude of more than 8.0, while the Lakshadweep-Minicoy island complex, situated on the Chagos-Laccadive ridge is moderately seismic. This broad picture of coastal and marginal seismicity is corroborated by the geodynamics of the northern part of the Indo-Australian Plate.Observations along the coastal areas during historic and recent times, however, confirm the absence of significant tsunamis, though very mild tsunami surges have occasionally been observed along the coastal areas of the Bay of Bengal. No active volcanoes are known to exist in the coastal areas.Water reservoirs situated near the marginal areas of the Peninsular Shield exhibit moderate to intense seismic activities, viz. Ukai, Bhatsa, Koyna, Parambikulam, Sholayar, Idduki, and Kinnersani.     

Metamorphism of the Koraput Alkaline Complex, Eastern Ghats Province, India—Evidence for reworking of a granulite terrane

An assemblage of predominantly metasedimentary rocks in the Eastern Ghats Province, India, underwent granulite facies metamorphism and deformation in early Neoproterozoic times, and was subsequently intruded by the Koraput alkaline complex. The intrusion was earlier believed to be syn- to late tectonic. The gabbroic core of the complex hosts nepheline-bearing syenitic dykes and veins. Following emplacement, magmatic amphibole within the syenites, and early orthopyroxene in feldspathic gneisses within the country rocks were retrogressed to biotite during pervasive solid-state deformation. Subsequent prograde metamorphism resulted in the formation of anhydrous assemblages at the expense of relict magmatic amphibole within the syenites, and metamorphic biotite in both the complex and the country rocks. Reactions reconstructed from textural observations indicate breakdown of biotite and amphibole to garnet + clinopyroxene ± orthopyroxene-bearing assemblages. Schreinemakers’ analysis on the relevant mineral associations suggests that heating was followed by loading of the region. This indicates thermal rejuvenation of the complex and the host granulites during an intracrustal orogeny that post-dates emplacement and cooling of the pluton. Available ages suggest that this event occurred in the mid-Neoproterozoic, and is probably unrelated to the amalgamation of the granulite belt with the Archaean Bastar/Dharwar craton.     

Hornblende-dehydration melting in mafic rocks and the link between massif-type charnockite and associated granulites,Eastern Ghats Granulite Belt,India

A massif-type (intrusive) charnockite body in the Eastern Ghats granulite belt, India, is associated with hornblende-bearing mafic granulite, two-pyroxene granulite and enderbitic granulite. The charnockite is characterised by pervasive gneissic foliation (S₁). This is axial planar to the folded layers of hornblende-bearing mafic granulite (F₁ folds), indicating that the granulite protoliths were present before the development of S₁. Two-pyroxene granulite and enderbitic granulite occur as lenticular patches disposed along the foliation and hence could be syngenetic to S₁. The tonalitic to granodioritic, metaluminous to weakly peraluminous compositions and relatively high Sr/Rb of the charnockite are consistent with its derivation by partial melting of a mafic protolith. Strong Y depletion, lack of Sr depletion and strongly fractionated REE patterns with high (La/Yb)_N ratio, but relatively lower HREE (Gd/Lu) fractionation with marked positive Eu anomalies, suggest major residual hornblende (as well as garnet), but not plagioclase, consistent with the hornblende dehydration melting in the source rocks. Such a residual mineralogy is broadly similar to those of some of the hornblende-bearing mafic granulite inclusions, which have compositional features indicative of a restitic nature. Quantitative modelling supports an origin for the charnockite melts by partial melting of a hornblende-rich mafic granulite source, although source heterogeneity is very likely given the rather variable trace element contents of the charnockite. The whole-rock and mineral compositions of the two-pyroxene granulites and enderbitic granulites are consistent with them representing peritectic phase segregations of hornblende-dehydration melting. A clockwise P-T path implies that melting could have occurred in thickened continental crust undergoing decompression.Editorial responsibility: T.L. Grove     

Crustal Magnetic Studies over Krishna-Godavari Basin in Eastern Continental Margin of India

Total field magnetic data were collected over the Krishna-Godavari basin covering 20, 000 sq.km with an average spacing of 8.5 km. This was mainly to study the long wavelength features related with the deep structures. Aeromagnetic map of the region compared well with the ground maps. The anomaly maps show a combination of NE-SW, NS/NNE-SSW and NW-SE trends. The anomalies of ground data are transformed to isolate the sources at different depths. The second vertical derivative and downward continuation maps bring out clearly the NE-SW and NS/NNE-SSW trends related to the coastal basin and Eastern Ghats implying that they are shallow. These are probably superposed on much deeper NW-SE trending structural features of Pre-Gondwana breakup as evidenced in the Horizontal Gradient of Pseudogravity and upward continuation maps. From the offshore magnetic data it appears that these trends extend up to the Ocean Continent Boundary. It is inferred that the deeper features are associated with rifting of Dharwar and Bastar cratons within the Indian plate, prior to the rifting of India from Gondwanaland. The superposed horst and graben structures are related to the formation of the pull-apart Krishna-Godavari basin as a result of rifting and drifting of India from Gondwanaland. These two structural features are associated with two different tectonic events.     

Fold-Thrust-Belt Structure of the Proterozoic Eastern Ghats Mobile Belt: A Proposed Correlation Between India and Antarctica in Gondwana

The Proterozoic Eastern Ghats Mobile Belt along the east coast of India shares a thrusted lower contact with the surrounding cratons. The thrust, known as the Terrane Boundary shear zone, is associated with two large lateral ramps resulting in a curved outline on the northwestern corner of the mobile belt. The Eastern Ghats Mobile Belt is divided into two lithotectonic units, the Lathore Group and the Turekela Group, based on their lithological assemblages and deformational history. On the basis of published data from a Deep Seismic Sounding (DSS) profile of the Eastern Ghats crust, the Terrane Boundary Shear Zone is considered to be listric in nature and acts as the sole thrust between craton and mobile belt. The Lathore and Turekela Groups are nappes. With this structural configuration the NW part is described as a fold thrust belt. However, the thrusting postdates folding and granulite metamorphism that occurred in the Eastern Ghats, as in the Caledonide type of fold thrust belt of NW Scotland. The Terrane Boundary Shear Zone is interpreted to be contiguous with the Rayner-Napier boundary of the Enderby Land in a Gondwana assembly.     

Aquifer characteristics and its modeling around an industrial complex,Tuticorin, Tamil Nadu,India: A case study

Anthropogenic pollution of shallow groundwater resources due to industrial activities is becoming a cause of concern in the east coastal belt of the state of Tamil Nadu, India. Integrated hydrogeological, geophysical and tracer studies were carried out in the coastal region encompassing an industrial complex. The objective has been to gain knowledge of aquifer characteristics, ascertaining groundwater movement and its flow direction, which would in turn reveal the possibility of contamination of groundwater regime and its better management. The results of multi-parameters and model study indicate that the velocity of groundwater flow ranges from 0.013 m/d to 0.22m/d in and around the industrial complex in upstream western part of the catchment and 0.026 m/d to 0.054m/d in the downstream eastern part, near the coast. These parameters are vital for the development of groundwater management scheme.     

Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south-eastern San Gabriel Mountains, southern California

Mid-Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south-eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc-silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic-intermediate magmatism and high-grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near-isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo-chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma^-1. The geotectonic evolution of the Cucamonga granulites is similar to mid-Cretaceous high- P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.     

Tectonics in the Cuddapah fold-thrust belt in the Indian shield,Andhra Pradesh,India and its implication on the crustal amalgamation of India and Rayner craton of Antarctica during Neoproterozoic orogenesis

Neoproterozoic orogenesis in East Antarctica and India led to the amalgamation of northern Prince Charles Mountains-Rayner complex of Antarctica with the Krishna Province of India along the present eastern coast of India with the development of ~990–900 Ma old fold-thrust belt. The frontal part of the fold-thrust belt [henceforth called the Cuddapah fold-thrust belt (CFTB)], recognized in the intercratonic, Palaeoproterozoic–Neoproterozoic Cuddapah Basin, includes two frontal thrust sheets carried by the eastern Velikonda and the western Nallamalai thrusts, along with a part of the undeformed foreland, constituting frontal part of a larger fold-thrust belt now fragmented and separated in different continents of Gondwanaland. Therefore, the intercratonic deformation now preserved in the Palaeoproterozoic–Neoproterozoic Cuddapah Basin is related to the collision of the Indian shield to the Antarctic block during the amalgamation of the Rodinia Supercontinent. CFTB is dominated by quasi-plastic deformational structures, representing exhumed deeper level fault-propagation folding related to the Velikonda thrust, while the Nallamalai thrust represents the forelandward thrust of the CFTB dominated by elastico-frictional deformation structures.     

Dehydration melting, fluid buffering and decompressional P–T path in a granulite complex from the Eastern Ghats, India

A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.     

The Himalayas

After splitting from Gondwanaland, India drifted northwards to collide with the Asian landmass about 40 million years ago. The intervening Tethys ocean was closed by northwards subduction beneath southern Tibet, and the collision created the Himalayan orogenic belt. Continuing northward movement of India at a rate of about 5 cm per year over the last 40 million years has caused it to indent Asia, and the resultant massive shortening is expressed by thrusting of the northern margin of India, by faulting and earthquakes in the Himalayas and China, by rifting and faulting in Tibet, and by the uplift of the Himalayas which is still continuing at rates of up to several millimetres per year.     

Juxtaposition of India and Madagascar: A Perspective

Proterozoic terrains in South India and Madagascar provide important clues in understanding the Gondwanaland tectonics, especially the assembly of this mega-continent during the Pan-African period. The Archaean terrains in both Madagascar and India are characterized by N-S trending greenstone belts occurring within gneissose granitic rocks in the northern part. Extensive development of K-rich granitic rocks of ca. 2.5 Ga is also characteristic in both areas. Such a broad age zonation of younger Dharwar (ca 2.6–3.0 Ga) in the north and the older Sargur (ca 3.0–3.4 Ga) in the south as in South India remains to be identified in future studies from Madagascar. The occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north-central terrain of Madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in South India. The Proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite-bearing khondalite in association with charnockitic rocks. While the Archaean-Proterozoic boundary is well defined in southern India by the Palghat-Cauvery or the KKPT shear zones as recently identified, this boundary is ill-defined in Madagascar due to extensive Pan-African overprinting, as well as the development of the Proterozoic cover sequence, the Itremo Group. There is also a possible general correlation between the Mesoproterozoic cover sequences in Madagascar and India, such as between the Itremo Group of west-central Madagascar and the Kaladgi and Cuddapah sequences of South India. The Pan-African granulite facies metamorphism of ca. 0.5 Ga extensively developed in both India and Madagascar is generally comparable in intensity and extent. P-T conditions and P-T-t paths also appear comparable, with the general range of ca. 700–1000°C and 6–9 kb, and near-isothermal decompressional paths. A-type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca. 500–800 Ma develop in both terrains, provide strong basis for the correlation of both terrains, and define a Pan-African igneous province within East Gondwanaland. Major shear zones in both continents are expected to play a critical role in the correlation, albeit are still poorly constrained. Detailed elucidation of the tectonic history of the shear zones, and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments.     

Southern Granulite Terrain, South of the Palghat-Cauvery Shear Zone: Implications for India-Madagascar Connection

The present paper correlates the southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages. Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks, the Madurai and the Kerala Khondalite belt, but all the blocks have similar lithologies with pelite—calc silicate rocks inter-banded with two pyroxene granulite bodies. These lithologies occur amidst an essentially charnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400–2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. The granulite metamorphism took place under temperatures of 800–1000°C and at pressures of 9.5 to 5 Kbar.The source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening.Many workers have linked Madagascar to southern India by connecting the Ranotsara shear either to the P-C shear zone or to the Achankovil shear zone, further south. The important factor is the lithologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to link the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikanal ranges and which merges with the southern margin of the P-C shear zone.However, north of the Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence (< 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malani sequences occurring further north east. The Rajasthan terrain has witnessed igneous intrusive activity at 1000–800 Ma. If we can broaden the area of investigations and include the above areas, the Madagascar-India connection can be better understood.