Journal of Southeast Asian earth sciences最新文献

The Arena Gneisses (AG), which is considered to form an eastern portion of the Wanni Complex (WC), is exposed at five doubly plunging synforms (arenas) in central Sri Lanka, near the boundary between the WC and the Highland Complex (HC). Structural investigation based on detailed field work on two of the arenas classified the following structural sequences from the earliest to the latest: Dn-2, Dn-1, Dn, Dn + 1, and Dn + 2 deformations. The Dn-2 structures include the major compositional banding, pinch-and-swell and boudinaged structures formed by compression normal to the banding. Mesoscopic intrafolial isoclinal to tight folds (Fn-2) were formed synchronologically with Dn-2 deformation. During the Dn-2 stage, the AG thrusted over the HC with top-to-the-southward movement. The asymmetric structures such as asymmetric foliation boudinaged structure, E-W-trending asymmetric folds (Fn-1b) and asymmetric megacrysts, and N-S-stretching lineations were formed by this simple shear deformation. This transport of the AG caused the N-S shortening and produced macroscopic gentle folds (Fn-1a) with mostly E-W-trending upright axial surfaces. During the Dn-1 stage, the eastward movement of the AG took place. This movement was associated with N-S-trending asymmetric folds (Fn-1c) and microfabrics observed in the AG, as well as the eastward vergences of related folds in the HC. N-S-trending upright linear folds (Fn) and related structures such as parasitic folds and axial surface foliations were formed by E-W shortening, which is normal to the axial surface of the macroscopic Fn fold, during the Dn deformational stage. Dextral shear deformation with vertical shear planes took place during Dn + 1 deformation. ENE-WSW- or NE-SE-trending folds (Fn + 1) were developed during Dn + 2 deformation. The southward thrusting of the AG indicates that the WC has displaced over the HC with top-to-the-southward movement. The eastward transportation of the AG and the HC may be due to the collision of the Vijayan Complex (VC) with the HC. Considering the above-mentioned tectonic elements with Sri Lankan geochronological data (e.g. Hölzl et al., 1994), the first thrusting event might have taken place around 700 Ma and the second displacement probably occurred around 570 Ma.

UPb zircon data from five igneous suites confirm previous studies that demonstrated widespread Pan-African magmatism in the Granulite Terrain of southern India. Ages determined here are ∼560 Ma for the Peralimala Granite and ∼555 Ma for the Kalpatta Granite, both north of the Palghat-Cauvery lineament, and ∼585 Ma for a charnockite in the Cardamom massif south of the lineament. Zircon from a pegmatite in the Kerala khondalite belt at Melankode yields an age of 512 Ma. Resetting of zircons in the 2500-Ma Arsikere Granite of the western Dharwar craton probably occurred at ∼450 Ma. These ages and the concentration of Pan-African granitic magmatism around the Indian portion of a broad region of granulite-facies metamorphism in East Gondwana demostrates generation of a restricted area of high temperature either above a rising plume or a zone of rifting. Mantle-derived fluids continued to move upward through the crust of southern India for at least 100 m.y. after the peak of magmatism, and the entire region was still cooling at 400 Ma.

Highly crystalline, coarse graphite flakes fill mesosopic shear zones and pegmatitic veins which cut across the regional penetrative fabric of granulite-facies supracrustals in the Kerala Khondalite Belt (KKB) of southern India. Carbon stable isotope analysis yield δ¹³C values of −8.2 to −12.4% for the shear-hosted graphites and −10.1 to −15.1% for the pegmatite-hosted graphites. These values are distinctly from the lighted carbon enriched values obtained from the disseminated graphite in the host gneisses (up to −32.1%). From the mode of occurrence and carbon isotopic composition, the graphite disseminations in the KKB supracrustals can be linked to the process of graphitisation of organic material trapped within sediments during regional high-grade metamorphism. A mechanism for the formation of shear- and vein-hosted graphites is discussed. It involves the influx of CO₂-rich fluids from a possible juvenile igneous source along structural pathways and precipitation of graphite under low oxygen fugacity conditions. Where the fluid pathways intersect impervious lithologies, fluid ponding and copious precipitation of graphite can be noticed at the lithologic contact. The graphite precipitation is considered to be synchronous with, and complementary to, the formation of arrested “incipient charnockites” elsewhere in the KKB, where upper amphibolite-facies gneisses transform to anhydrous granulite-facies assemblages through the influx of CO₂-rich fluids.

Green vanadium grossulars occur as porphyroblasts in calc-silicate-graphite gneiss associated with marble, pelitic-psammitic gneiss and granitoid gneiss in the Mozambique metamorphic belt, south Kenya. Calc-silicate-graphite gneiss contains scapolite, vanadian zoisite, vanadian diopside, vanadian sphene and vanadian magnetite, in addition to vanadium grossular. The vanadium grossular porphyroblasts are mantled by a kelyphitic rim that consists mainly of symplectic intergrowth of fine-grained scapolite, vanadium grossular, vanadian diopside, vanadian sphene, vanadian magnetite, plagioclase, calcite and quartz. Vanadian minerals (vanadian muscovite, vanadian zoisite, vanadian sphene and vanadian rutile) also occur in the marble. The occurrence of scapolite and the enrichment in vanadium suggest that the protolith of calc-silicate-graphite gneiss was evaporite or related sediment. The mode of occurrence of vanadium grossular in Kenya is quite similar to that in the Sør Rondane Mountains, East Antarctica. The vanadium-grossular-bearing gneisses in Kenya might be correlated with those in Antarctica, since the Sør Rondane Mountains had continued from the Mozambique metamorphic belt before the breakup of Gondwanaland.

The Eastern Ghats on the east coast of India is a largely granulite terrain but also exposes granites, migmatites, anorthosites and alkaline rocks. This granulite belt has had a prolonged history of mountain building from late Archaean to late Proterozoic. During this long period the Eastern Ghats mobile belt witnessed repeated folding and possibly polycyclic metamorphism. Some recent findings suggest breaks between orogenic cycles and a proterozoic reworking of Archaean granulites. Extreme-temperature crustal metamorphism under fluid-absent conditions and crustal anatexis in huge thickness of pelitic to psammitic protoliths producing leptynites are some of the important results of recent investigations of the Eastern Ghats mobile belt. Different generation of charnockites are present in the Eastern Ghats belt, but charnockitisation of granitic gneisses is yet to be documented. Some apparently nascent growths, the patchy charnockites in the Chilka area are shown to be relict of older charnockitic rocks that suffered granulite-facies metamorphism and attendant migmatisation.

The sapphirine-bearing granulites exposed around the Kodaikanal region of the Madurai block, southern India, present a variety of mineral parageneses involving garnet, spinel, sillimanite, cordierite, orthopyroxene, phlogopite, potash feldspar and plagioclase. interpretation of multiphase reaction textures in conjunction with mineral chemical data and topology in the (FM)AS system is consistent with the main sapphirine-forming reactions: orthopyroxene + sillimanite = sapphirine + cordierite phlogopite + sillimanite = sapphirine + cordierite + K-feldspar + vapour Mg-tschermak = sapphirine + cordierite. The P-T evolution of these sapphirine granulites has been constrained through the use of conventional geothermobarometry, internally consistent TWEEQU programme and thermodynamically calibrated MAS equilibria. The P-T estimates define a retrograde trajectory with substantial decompression of c.4 kbar from P-T_max of c.8 kbar at c.800°C. On the basis of the available evidence for a Pan-African granulite-facies event in the Madurai and Trivandrum blocks, the emerging concept of East Gondwana assembly is endorsed.

Orissa possesses an excellent record of geological history spanning most of the geologic time from Archaean to the Quaternary. It has most of the typical lithologies and many tectonothermal events preserved in the various rock groups. Three distinct crustal blocks could be identified: two cratonic blocks and a mobile belt separated from each other by deep-seated regional fault boundaries. These are the north Orissa craton (NOC), the west Orissa craton (WOC) and the Eastern Ghats granulite belt (EGB). The fault boundaries separating them are identified as the north Orissa boundary fault (NOBF) and the west Orissa boundary fault (WOBF). The NOBF fault running along Mahanadi Valley could be termed the ‘Mahanadi rift’. The NOC contains extensive occurrences of low-grade folded banded iron formations (BIFs), granite intrusives and undeformed volcano-sedimentary assemblages belonging to the Archaean to early Proterozoic times. They are succeeded by medium-grade folded Proterozoic limestone-bearing sequences. The WOC craton is underlain by extensive occurrences of Archaean granites and undeformed Proterozoic limestone-bearing platform sediments. Small occurrences of Archaean BIFs and greenstones have also been noticed. The EGB consists of high-grade granulite-facies rocks such as the khondalites, charnockites, basic granulites, migmatites and augen gneisses. It has been considered as a ‘mobile belt’ during the middle Proterozoic Era. A distinct early Neoproterozoic (ca. 1000 Ma) charnockite event has been recorded in this belt. Close to the faulted boundaries of the crustal blocks are seen occurrences of anorthosites, alkaline rocks and chromite-bearing ultramafics. The rocks in each crustal block show evidence of multiphase tectonothermal history. Similarity of lithology, tectonothermal events and major rift features, e.g. the Mahanadi rift, place Orissa close to Eastern Antarctica.

The Bhopalpatnam and Kondagaon granulite belts (BGB, KGB) occur surrounding the Bastar craton of central India. This paper deals with the geology, mineralogy and the fluid characteristics of these two belts. The geology of the two belts indicate that they abound in metasedimentary swathes. The metamorphic P-T conditions of the BGB range from 6 to 9 kbar at temperatures of 750°C, whilst those of the KGB vary from 4 to 6 kbar at temperatures of 700°C. The BGB shows an IBC path, while the KGB exhibits a dominant ITD path. These trends, based on mineral chemistry, are corroborated by fluid-inclusion studies. The lithologies of the BGB exhibit high-density CO₂ inclusions that represent the remnants of peak metamorphism. In contrast, the CO₂ inclusions of the KGB are of low density, indicative of post-peak conditions. The studies suggest that the CO₂-rich fluids may not have come from the supracrustal sediments. An external source, possibly underplated basalt, could have supplied the heat and the supercritical fluids. Based on lithological similarities, it can be stated that the BGB is an extension of the late Archaean Karimnagar granulite belt. The important problem which is still to be solved is the junction of the late Archaean BGB with the Eastern Ghats granulite belt.

Leptynites occur with minor bands of basic granulites, charnockites, calc-silicate rocks, quartzites and khondalites in the Visakhapatnam area in the Eastern Ghats granulite terrane (EGGT), India. Interbedded grey and cream leptynite types are distinguished based on field relations and geochemistry. The mineralogical and geochemical evidences favour a metasedimentary origin for leptynites. The total REE, LREE/HREE and Eu-Eu^∗ data indicate their derivation from the post-Archaean granodioritic upper crust comprising large proportions of impure greywackes with K-rich granitic components.

The East Gondwana crustal fragments, namely southern India, Sri Lanka, Madagascar, East Antarctica and Western Australia, preserve evidence for a prominent Pan-African felsic magmatic event represented by feldspathoidal syenites, alkali granites and syenites emplaced proximal to fault lineaments. In Peninsular India, late or post-kinematic alkaline plutons ranging in age from 864 to 600 Ma characterize the Eastern Ghats belt, whereas a strong Pan-African imprint is provided by 550–750 Ma alkali granites and syenites in the southern granulite segment. In Sri Lanka, alkali granites occur in all major geologic units, with radiometric ages in the range of 550–760 Ma. In Madagascar, alkali granites and syenites with late Proterozoic to Palaeozoic ages are reported. Felsic magmatic activity in East Antarctica is characterized by post-tectonic granitic and syenitic plutons in Enderby Land and Queen Maud Land, showing an age range of 550–770 Ma. Pan-African felsic plutons in Western Australia define ages of 550–570 Ma. This paper summarizes the petrologic, geochemical and geochronologic characteristics of felsic plutons in the various East Gondwana fragments. Many of the intrusives represent anorogenic A-type magmas generated in rift-related environments of high heat flow and abundant volatile activity, correlative with an extensional tectonic regime and probably including melts generated from both upper mantle and lower crustal sources. This paper identifies a major Pan-African felsic magmatic province in East Gondwana, which, in association with their petrogenetic significance, is considered to be suggestive of a geodynamic signature of East Gondwana during the Pan-African period.