Yoshikuni Hiroi

Evidence for prograde metamorphic evolution of Sri Lankan pelitic granulites, and implications for the development of continental crust

Petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined Highland Complex in Sri Lanka. It includes the occurrences of relict kyanite±quartz, staurolite, corundum±kyanite, hercynite+kyanite and sapphirine+kyanite±spinel within garnet in the sillimanite-quartz-rich rocks. Of special interest is the mineralogy of khondalites (graphitic garnet-sillimanite-K-feldspar-quartz gneisses), the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks. However, relict hydrous minerals such as AlTi-rich (more than 58 wt% Al2O3 and more than 1 wt% TiO2) staurolite and extremely Ti-rich (up to 9.8 wt% TiO2) biotine occur as inclusions within garnet. The local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite-breakdown reactions which may have progressed as temperature increased and pressure decreased: staurolite+quartz=kyanite+garnet+H2O; staurolite=corundum+kyanite+garnet+H2O; staurolite=kyanite+hercynite+garnet+H2O; staurolite=sillimanite+hercynite+garnet+H2O. Some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the Highland Complex. Moreover, garnet in the Highland Complex pelitic granulites occasionally preserves growth zoning, though modified slightly to extensively by post-growth volume diffusion. Based on the evidence, a clockwise prograde P-T path of evolution is inferredfor meta-sedimentary granulites belonging to the Highland Complex. On the other hand, no evidence for prograde metamorphism has so far been obtained in “lower grade” rocks of the Wanni Complex to the northwest of the Highland Complex. Such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the Sri Lankan basement rocks based virtually on the distinctly different neodymium model ages. It suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the Highland Complex rocks, that is during decompression and heating, probably by the lower crustal lateral flow after tectonic overthickening and delamination. The clockwise prograde P-T-t path followed by the meta-sedimentary granulites of the Highland Complex is in marked contrast with the near isobaric cooling path for meta-igneous granulites suggested by some authors. It is proposed that both types of P-T-t paths albeit for different rock types, are possible and resulted from diverse deep crustal processes, such as underplating of continental crust by sediments and magmas or near convergent plate boundaries.

Phase petrology of eclogites and related rocks from the Motalafjella high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance

Abstract A Caledonian eclogite suite is associated with lower-grade high-pressure rocks in Motalafjella, Oscar II Land, central-western Spitsbergen. A variety of mineral assemblages occur in the suite, depending on the bulk-rock composition; omphacite-paragonite-glaucophane-epidote-garnet in eclogites, jadeite-glaucophane-paragonite-quartz in siliceous schists, and MgFe chloritoid-paragonite-epidote-garnet in pelitic schists. Petrologic and paragenetic studies are presented to characterize this unique set of mineral assemblages in relation to other eclogite suites. Mineralogical data, mainly the distribution of Fe and Mg between coexisting clinopyroxene and garnet, and the existence of impure jadeite+quartz assemblage, give approximately 580–640°C and 18–24 kbar. New criteria for the eclogite classification are derived based on the mineral assemblages and a model petrogenetic grid. The Motalafjella eclogites associated with the glaucophane-epidote assemblage belong to the medium-temperature portion of the eclogite facies.

Geodynamic Evolution of East Antarctica: A Key to the East–West Gondwana Connection

Geological correlations of East Antarctica with adjoining continents have been puzzling geologists ever since the concept of a Gondwana supercontinent surfaced. Despite the paucity of outcrops because of ice cover, difficulty of access and extreme weather, the past 50 years of Japanese Antarctic Research Expeditions (JARE) has successfully revealed vital elements of the geology of East Antarctica. This volume presents reviews and new research from localities across East Antarctica, especially from Dronning Maud Land to Enderby Land, where the geological record preserves a history that spans the Archaean and Proterozoic. The reviews include extensive bibliographies of results obtained by geologists who participated in the JARE. Comprehensive geological, petrological and geochemical studies, form a platform for future research on the formation and dispersion of Rodinia in the Mesoproterozoic and subsequent assembly of Gondwana in the Neoproterozoic to Early Palaeozoic.

Magmatic evolution and tectonic setting of metabasites from Lützow-Holm Complex, East Antarctica

Abstract Metabasites from the Lützow-Holm Complex, East Antarctica, are the equivalent of metamorphosed ultramafic and mafic rocks with ultrabasic to intermediate compositions, which occur as layers and blocks in the quartzo-feldspathic or metasedimentary gneisses. Field occurrences and whole-rock geochemistry suggest that the ultramafic rocks are all cumulitic protoliths, whereas the mafic rocks are mostly basaltic protoliths including some cumulates. Moreover, in a regional context, the geochemistry of metabasites shifts from island arc to ocean-floor affinities in a southwesterly direction from the Prince Olav Coast to the Lützow-Holm Bay area. Neodymium isotopic data suggest that the metamorphic rocks from the Prince Olav Coast and the northern Lützow-Holm Bay areas were derived from immature continental crust formed by active Mesoproterozoic crustal growth, whereas those from the southern Lützow-Holm Bay area were derived from mature continental crust and oceanic crust of older age. Thus, these results suggest that the Lützow-Holm Complex includes lithological units with various origins and ages that were amalgamated by multiple subduction, and underwent high-grade metamorphism as a result of the final collision of East and West Gondwana during the Pan-African orogeny.

Origin of xenocrystic garnet and kyanite in clinopyroxene-hornblende-bearing adakitic meta-tonalites from Cape Hinode, Prince Olav Coast, East Antarctica

Abstract Xenocrystic garnet and kyanite, in addition to clinopyroxene and rare orthopyroxene, are newly found to occur in middle Proterozoic slightly metamorphosed adakitic trondhjemites and tonalites (meta-tonalites) at Cape Hinode on the eastern Prince Olav Coast in the latest Proterozoic–Early Palaeozoic Lützow-Holm Complex, East Antarctica. Textural and compositional features of garnet and kyanite suggest that these minerals formed most probably as restite phases of partial melting of mid-ocean ridge basalt (MORB) between 15 and 20 kbar pressure, and were entrained by the tonalitic magmas, which underwent fractional crystallization upon ascent to form cumulates that were also entrained and metamorphosed to basic–intermediate granulite blocks. Available geochronological data for the meta-tonalites indicate that all these events including MORB formation took place in the middle Proterozoic. The meta-tonalites and associated basic, calc-silicate, and pelitic rocks were emplaced as an allochthonous block in the Lützow-Holm Complex at the waning stage of its main regional metamorphism, most probably as a part of the final amalgamation of East and West Gondwana into the Gondwana supercontinent.

Clinopyroxene exsolution in wollastonite from Namaqualand granulite, South Africa

Abstract Chemical and crystallographic properties of clinopyroxene exsolution in wollastonite are described from metamorphosed calc-silicate granulite, Namaqualand, South Africa. The wollastonite is Ca1.96Fe0.01Al0.01Si2.01-O6 belonging to space group P21/a (2M polytype) and the clinopyroxene is Ca0.99Mg0.75-0.80Fe0.17-0.21Na0.02Al0.03Si1.99-2.00O6 belonging to C2/c. An electron backscattered diffraction investigation suggests that the clinopyroxene lamellae elongated along [11̅1] lie on (120) and (100) of the wollastonite-2M, and [11̅0] of both lamellae are parallel to [001] of the wollastonite-2M. The formation of the exsolution probably results from the relatively high peak metamorphic temperature (800-860ºC) of the Namaqualand granulite and its slow cooling rate.