Journal of Southeast Asian earth sciences最新文献

East Gondwana incorporates a collage of polymetamorphic terrains with long-lived tectonic histories from the Early Archaean to the Neoproterozoic. The oldest cratonic areas have been identified in South India (north of the Palghat-Cauvery shear zone) and East Antarctica (the Napier Complex). These terrains are remnants of an East Gondwana craton that underwent initial crustal growth during the Early Archaean and granulite-facies metamorphism at ∼2.5 Ga. Both were virtually unaffected by the Pan-African orogeny (1.1-0.5 Ga). In contrast, Proterozoic terrains were subject to high-grade metamorphism during the Pan-African event. On the basis of published Nd model ages, a direct correlation can be made between southern Madagascar (south of the Ranotsara shear zone), southern India (the Madurai Block and Kerala Khondalite Belt) and the Highland/Southwestern Complex of Sri Lanka, which comprise a Later Archaean-Palaeoproterozoic (3.2-2.0 Ga) mobile belt that may extend eastwards into East Antarctica. The youngest period of crustal growth in East Gondwana has been recognised at 1.5-0.8 Ga from isotopic studies of the Mozambique Belt of East Africa, the Vijayan Complex of Sri Lanka and the Yamato-Belgica Complex/Sør Rondane Mountains of East Antarctica. Small slivers of terrain of intermediate age (1.9-1.2 Ga) have been recognised in South India (Achankovil metasediments) and Sri Lanka (Wanni Complex) that may represent mixed-age contributions to clastic sedimentary basins.

Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures <1000°C and were thus in thermal equilibrium with the country rock granulites during peak metamorphism. Comparison with experimental data on similar bulk compositions constrains the depth of emplacement of the magmas at 30–35 km. Geochemical characteristics indicate that the mafic magmas are essentially similar to continental flood basalts and have thus been generated in an extensional set-up. The apparent clockwise trajectory recorded in the Anakapalle granulites was produced by extension of the crust of near-normal thickness with concommitant basic magmatism.

The occurrence of the Holenarasipur greenstone belt is explained by accretionary process at a trench. A sequence of amphibolite (MORB), chert, and banded iron formation associated with komatiitic amphibolite (oceanic island material) is an Archaean analogue to the modern oceanic crust grown through the migration from an active ridge to a trench. At a trench, slices of such a sequence mixed with or became covered by turbidite from land mass (quartzite and conglomerate) to form an accretionary complex. Available SHRIMP data suggest the synchronous formation of nearby tonalite-trondhjemite-granodiorite (TTG) with the greenstone belt by melting of subducting oceanic plate. The Dharwar craton has been built up stepwise: (1) Sargur stage—ocean-ocean collision to form oceanic island arc with TTG and an accretionary complex (older greenstone belt), followed by the collision of such arcs to form mini-continents at the early to middle Archaean (unit 3.0 Ga); (2) Dharwar stage—the amalgamation of mini-continents with newer accretionary complexes (younger greenstone belt) under the same subduction polarity caused by the change from two layered to whole mantle convection with the progressive cooling of the earth (3.0−2.5 Ga). The cessation of TTG activity was also due to the cooling of the earth. Tectospheric peridotite keel formed with TTG has acted as a thermal insulator to stabilize the Dharwar craton after the Archaean.

Recent geochronologic data allow us to propose a new characterization of Precambrian granulite terranes in Peninsular India. The Archean granulite terranes, including the Godavari Granulite Belt (GGB) along the Godavari Valley and the Nilgiri-Madras Granulite Belt (NMGB) along the southern fringe of the Dharwar Craton and north of the Palghat-Cauvery Shear Zone, are characterized by metamorphic events at ca. 2.8–3.0 Ga and intense granitic activity at ca. 2.5 Ga associated with middle- to high-grade metamorphism. The Proterozoic granulite terranes include the Eastern Ghats Granulite Belt (EGGB) along the Bengal Bay Coast of India, the Periyar-Madurai Granulite Belt (PMGB) south of the Palghat-Cauvery Shear Zone and north of the Achankovil Lineament, and the Trivandrum Granulite Belt (TGB) south of the Achankovil Lineament. The Proterozoic granulite belts are characterized by model Nd ages (T_DM) of mostly ca. 2.0–3.0 Ga with local 1.3–1.8 Ga. Intense deformation and regional high-grade metamorphism predates emplacement of A-type granitic plutons at ca. 550–740 Ma and the last granulite metamorphism took place at ca. 550 Ma. Both GGB and NMGB have similar geochronologic characteristics to the Napier Complex of East Antarctica in that they suffered ca. 2.8–3.0 Ga and ca. 2.5 Ga tectonothermal events. EGGB, PMGB and TGB have similar ranges in Nd T_DM ages as the Rayner and the Lützow-Holm complexes in East Antarctica. They are also similar to the Sri Lankan Precambrian, when the three Precambrian units in Sri Lanka are not differentiated and mixed altogether. In this assembled East Gondwana, late Archean granulite terranes form a continuous belt from NMGB to GGB through the Napier Complex, forming a horseshoe-shaped belt surrounding the Dharwar Craton. The Paleoproterozoic-Mesoproterozoic terranes form a broad belt continuing from EGGB to PMGB through the Rayner and the Lützow-Holm complexes. This belt forms a major part of the Mesoproterozoic Circum East Antarctic Mobile Belt surrounding East Antarctica, which is important for the assembly of East Gondwana. Within the Proterozoic terrains, signatures in the distribution of T_DM ages facilitate the differentiation of the Paleoproterozoic and Mesoproterozoic terranes which have distinct distributional characteristics.

Orthopyroxene granulites (charnockites, enderbites and associated rocks) form one of the major lithological units in Rayagada, in the north-central sector of the Eastern Ghats Granulite Belt of Peninsular India. Petrographic features show common occurrences of clinopyroxene lamella within orthopyroxene host, although reverse relations have also been observed in some cases. Two types of coronal garnet with different chemical compositions have been noted: one at the interface of orthopyroxene and plagioclase, and the other rimming magnetite, also at the contact of plagioclase. Garnet-orthopyroxene thermometry from these rocks show temperature variations from 840 to 700°C, whereas two-pyroxene and garnet-biotite thermometry yield temperatures around 750 and 700°C, respectively. Garnet-orthopyroxene-plagioclase-quartz barometry ranges from 8.1 to 6.8 kbar. A near-isobaric cooling from a thermal maxima of ∼840°C, followed by decompression is characteristic of the deduced pressure-temperature (PT) path. This shows significant similarities with the PT path derived from associated metapelites. Field and petrographic features collectively suggest that these orthopyroxene granulites have suffered at least the second deformation event (D₂), which is the dominant ductile deformation of the area. This event presumably took place around 1000 Ma and might be coeval to the mid-Proterozoic events of East Antarctica and other fragments of Gondwanaland.

This paper reports several new localities of wollastonite- and scapolite-bearing calc-silicate assemblages from the granulite-facies supracrustal Kerala Khondalite Belt (KKB), southern India. Based on mineralogy, these calc-silicate rocks are classified into four types: Type I, lacking wollastonite and grossular; Type II, wollastonite-bearing but grossular-absent; Type III, wollastonite- and grossular-bearing; and Type IV, dolomitic marbles. Detailed petrographic studies reveal a variety of reaction textures overprinting the polygonal granoblastic peak metamorphic assemblages in these rocks. The Type II calc-silicate rocks preserve reaction textures, including meionite breaking down to anorthite-calcite-quartz, wollastonite breaking down to calcite-quartz and meionite-quartz symplectites after K-feldspar and wollastonite. Type III calc-silicate rocks have porphyroblastic and coronal grossular. Grossular-quartz coronas separating wollastonite and anorthite and the development of grossular within the anorthite-calcite-quartz pseudomorphs of meionite form important retrograde reaction textures in this type. In Type IV dolomitic marble assemblages, meionite forming in grain boundaries of calcite and feldspars, forsterite rimmed by diopside-dolomite and the formation of grossular in feldspar-rich zones are the important textures. Calculated partial petrogenetic grids in the CaOAl₂O₃SiO₂CO₂ system are used to deduce the pressure-temperature-fluid evolution of the calc-silicate rocks. The Type II assemblages provide CO₂ activity estimates of > 0.5, with a peak metamorphic temperature of about 790°C. Initial cooling followed by later CO₂ influx can be deduced from reaction modelling in these calc-silicate rocks. Type III assemblages are characterized by internal fluid buffering throughout their tectonic history. The formation of coronal grossular indicates an initial cooling from peak metamorphic temperatures of about 830°C deduced from vapour-absent meionite and grossular equilibria. Type IV marble assemblages also indicate internal fluid buffering followed by localized CO₂ influx. Overall, the calc-silicate rocks of the KKB define peak metamorphic temperatures in the range of 790–850°C, with an internally buffered fluid composition during the peak conditions. Initial cooling was followed by localized carbonic fluid influx that also post-dated decompression deduced from other rock types in the KKB.

The Baradangua alkaline complex on the southern bank of the Brahmani river, Orissa, strikes E-W with steep southerly dips, in conformity with the regional trend of the litho units of the Eastern Ghat Mobile Belt of this sector. The nepheline syenite, a hypersolvus rock, comprises potash feldspar, nepheline, plagioclase and calcite, with biotite as the dominant mafic mineral. The agpaitic index [$\text{(}\text{Na}{}_2\text{O}\text{+}\text{K}{}_2\text{O}\text{)}\text{Al}{}_2\text{O}{}_3$ mol prop] ranges from 0.43 to 0.74 in the nepheline syenites, indicating their miaskitic nature. Field, petrography and geochemistry suggest a magmatic origin for nepheline syenites, the differentiation pattern being reflected by the distribution of the trace elements.

Major and rare-earth elements (REE) of banded iron formations (BIFs) and associated amphibolite from the Sargur belts, the oldest schist belts in the Dharwar craton, were determined by X-ray fluorescence and inductively coupled plasma mass spectrometry (ICP-MS). The chondrite-normalized REE patterns of BIFs are light REE-enriched with a striking positive Eu anomaly, resembling those of modern hydrothermal solutions from the mid-oceanic ridge. Amphibolite is flat and chondritic in the REE plot. The $\text{Al}{}_2\text{O}{}_3\text{TiO}{}_2$ ratio of BIFs is about the same as that of amphibolite and is different from that of terrigenous clastics. These facts suggest that the BIFs were of hydrothermal origin and had a genetic relation to amphibolite, which may have originated from the Archaean mid-oceanic ridge basalt. Subtle negative or no Ce anomaly of BIFs indicates that contemporary seawater was less oxic than today.