Diana Plavsa

Detrital zircons in basement metasedimentary protoliths unveil the origins of southern India

Coupled U-Pb and Hf isotopic analysis of detrital zircons from metasedimentary rocks of the Southern Granulite terrane (India) provides provenance information that helps unravel their paleotectonic position before Gondwana amalgamated. The metasedimentary packages of the Salem block (southernmost extension of Dharwar craton) record a restricted juvenile late Archean to early Paleoproterozoic (2.7–2.45 Ga) source provenance and epsilon Hf values between +0.3 and +8.8. Similar late Archean juvenile crust is found throughout the Dharwar craton and represents a likely source for the Salem block metasedimentary rocks. By contrast, the metasedimentary rocks of the Madurai block (south of the Salem block) show a predominantly Archean to Paleoproterozoic provenance (3.2–1.7 Ga) in the northern part of the Madurai block and a largely late Mesoproterozoic to Neoproterozoic provenance (1.1–0.65 Ga) in the southern part of the Madurai block. Collectively, the Madurai block metasedimentary rocks record a mixture of reworked Archean and Paleoproterozoic sources, as well as juvenile Paleoproterozoic, late Mesoproterozoic, and evolved Neoproterozoic sources. These detrital signatures best fit the combined basement ages of the Congo-Tanzania-Bangweulu block and central Madagascar (Azania), thus linking the tectonic evolution of the southernmost tip of India to these domains throughout much of the Proterozoic. The diachroneity of metamorphic ages obtained from the rims of Madurai block detrital zircons attests to their poly-metamorphic history that is different from that of the Salem block. The contrasting metamorphic and depositional histories between the Salem and Madurai blocks place them on opposite sides of the Mozambique Ocean until the latest Neoproterozoic when they came together to form Gondwana.

The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600–500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740–550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1–D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1–D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550–500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the Ediacaran/Cambrian.

Zircon U-Pb ages and Hf isotopic systematics of charnockite gneisses from the Ediacaran–Cambrian high-grade metamorphic terranes, southern India: Constraints on crust formation, recycling, and Gondwana correlations

The Southern Granulite terrane, southern India, comprises a vast exposure of deep crust and forms a key region in the reconstruction of the Gondwana supercontinent. An E-W−trending crustal-scale shear zone, the Palghat-Cauvery suture zone system, which formed during the late Neoproterozoic−Cambrian (0.75−0.50 Ga), marks a prominent terrane boundary separating terranes with a predominantly late Neoarchean (ca. 2.5 Ga) regional granulite metamorphism to the north from those with an Ediacaran−Cambrian (0.63−0.50 Ga) regional granulite metamorphism to the south. Focusing on the younger granulite metamorphic domains, we present here new zircon U-Pb ages and Hf isotopic compositions for 11 charnockite orthogneisses from the Madurai, Trivandrum, and Nagercoil blocks and contribute to the resolution of the age of their magmatic protoliths. This study shows that the charnockite orthogneisses south of the Palghat-Cauvery suture zone relate to a minimum of four distinct episodes of felsic magmatism centered at: ca. 2.62−2.46 Ga, ca. 2.05−1.84 Ga, ca. 1.0−0.9 Ga, and ca. 0.80−0.76 Ga, pertaining to the Siderian, Orosirian, and Tonian Periods. Hafnium isotope analyses of zircon grains from the charnockite gneisses suggest that the protoliths of the ca. 2.05−1.98 Ga gneisses from the Trivandrum and Nagercoil blocks and the ca. 1.0−0.9 Ga gneisses along the southeastern Madurai block involved a significant juvenile magma component, while the protoliths of charnockite gneisses elsewhere in the Madurai block formed mainly through recycling of older crust up to ca. 3.2 Ga. A regional granulite-facies metamorphic imprint during the Ediacaran−Cambrian marked an advanced stage in the amalgamation of the Madurai, Trivandrum, and Nagercoil blocks into the East African orogen and its collision with the Dharwar craton.

Petrology and geochemistry of amphibolites and greenschists from the metamorphic sole of the Muslim Bagh ophiolite (Pakistan): implications for protolith and ophiolite emplacement

Metamorphic sole rocks are exposed beneath both the Jang Tor Ghar Massif (JTGM) and Saplai Tor Ghar Massif (STGM) of the Muslim Bagh ophiolite. The sole rocks comprise the basal mylonitic part of the ophiolite peridotites and the sub-ophiolitic metamorphic rock series showing inverted metamorphic gradients. The latter mainly consist of garnetiferous amphibolites, amphibolites and greenschists. The mineralogy of the amphibolites (hornblende + plagioclase ± quartz ± biotite ± epidote ± apatite ± opaque) and garnet amphibolites in the metamorphic sole rocks of the Muslim Bagh ophiolite is similar except for the presence of garnet in the latter. Greenschists contain minerals such as chlorite + plagioclase + epidote ± actinolite ± quartz ± opaques. The mineral assemblages of these rocks suggest that they are meta-basites. Geochemical analyses indicate that the garnetiferous amphibolites are metamorphosed tholeiitic to alkaline basalts, akin to ocean island basalts (OIB). By contrast, the amphibolites and greenschists have geochemical signatures akin to mid-oceanic ridge basalts (MORB). Basalts of OIB type are also found in the hyaloclastite-mudstone unit (Bhm), while the MORB-type basalts are found in the basalt-chert unit (Bbc) of Bagh complex underlying the ophiolite nappe. Here, we interpret an early stage OIB-type basalt accretion to the base of the obducted plate associated with extrusion of volcanic rocks in the Bhm unit of Bagh complex followed by amphibolite facies metamorphism. During the later stage of the advancing ophiolitic thrust sheet, MORB-like basalts, such as those found in the Bbc unit of the Bagh complex, are underplated and metarmophosed to greenschist facies with subsequent accretion of the entire sequence of the Muslim Bagh ophiolite and the Bagh complex onto the Indian Platform sediments.