Mesoproterozoic ophiolitic mélange from the SE periphery of the Indian plate: U–Pb zircon ages and tectonic implications

The Kanigiri mélange within the Proterozoic Nellore–Khammam schist belt in southern Peninsular India includes ophiolitic fragments that represent the remnants of an oceanic plate. The ophiolitic units were accreted along a NE-trending suture that juxtaposes the Proterozoic Eastern Ghats Granulite Belt (EGGB) against the Archean Nellore Schist Belt of the Dharwar craton. The ophiolite components in the Kanigiri mélange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature. We report laser ablation-ICP-MS age data and REE geochemistry of zircons from the gabbro and granite. The zircons from both gabbro and granite show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. Zircon REE abundances and normalized patterns show little intersample and intrasample variations. U–Pb dating of the zircons reveals prominent Mesoproterozoic ages for the plagiogranite, with the ca.1.33 Ga age of the Kanigiri ophiolitic mélange offering important clues for arc–continent collision during the final stages of amalgamation of the Columbia-derived fragments within the Neoproterozoic supercontinent assembly.     

Transition from shoshonitic to adakitic magmatism in the eastern Pontides,NE Turkey: Implications for slab window melting

The formation of the eastern Pontides orogenic belt has been widely assigned to a northward subduction of the Neotethyan oceanic slab during the late Mesozoic–Cenozoic. Here we provide an alternate model based on new geological, geochemical and isotopic data. The magmatic activity in the far south of the belt started in the early Campanian with shoshonitic trachyandesites and associated pyroclastics. This sequence is covered by the late Campanian–early Maastrichtian reefal limestones and another stage of high-K volcanism represented by analcimized leucite-rich ultrapotassic rocks of the Maastrichtian–early Paleocene (?) ages. The shoshonitic and ultrapotassic rocks, with K₂O contents ranging from 0.26 to 6.95 wt.%, display broadly similar rare earth and multi-element distribution patterns. Both rock types are enriched in LILE and LREE and depleted in HFSE (Nb, Ta and Ti), suggesting a subduction-enriched mantle source for the magma generation. Subsequently, during the late Paleocene, a stage of acidic magmatism (SiO₂ of 53.25–73.61 wt.%) that shows adakitic geochemical characteristics including high Sr/Y (46–416) and La/Yb (11–51) and low Y (2.6–12.2 ppm), is documented characterized by melting of a mafic source such as the MORB crust with garnet in the residue. The adakitic magmatism began at ~ 56 Ma and migrated toward the north through time, culminating with porphyritic andesites (~ 47 Ma) that were emplaced in the Gumushane–Bayburt line and its vicinity. North of this line, coeval magmas show typical calc-alkaline nature and continued to develop toward further north until the middle to late Eocene. Based on the spatial and temporal variations in the magmas generated in the eastern Pontides orogenic belt, we propose a new geodynamic model to explain the tectonomagmatic evolution of these rocks and correlate the adakitic magmatism to ridge subduction and slab window process within a south-dipping subduction zone. Our model is in contrast to the previous proposals which envisage partial melting or delamination of thickened lower continental crust due to the collision in the south during the Paleocene–Eocene.     

Crustal architecture beneath Madurai Block,southern India deduced from magnetotelluric studies: Implications for subduction–accretion tectonics associated with Gondwana assembly

The Madurai Block in southern India is considered to represent the eroded roots of an arc-accretionary complex that developed during the subduction–collision tectonics associated with the closure of the Mozambique Ocean and final suturing of the crustal fragments within the Gondwana supercontinent in the Late Neoproterozoic–Cambrian. Here we present a magnetotelluric (MT) model covering the main collisional suture (Palghat–Cauvery Suture Zone) in the north into the central part of the Madurai Block in the south comprising data from 11 stations. Together with a synthesis of the available seismic reflection data along a N–S transect further south within the Madurai Block, we evaluate the crustal architecture and its implications on the tectonic development of this region. According to our model, the predominantly south dipping seismic reflectors beneath the Madurai Block define a prominent south-dipping lithological layering with northward vergence resembling a thrust sequence. We interpret these stacked layers as imbricate structures or mega duplexes developed during subduction–accretion tectonics. The layered nature and stacking of contrasting velocity domains as imaged from the seismic profile, and the presence of thick (>20 km) low resistivity layers ‘floating’ within high resistivity domains as seen from MT model, suggest the subduction of a moderately thick oceanic crust. We identify several low resistivity domains beneath the Madurai Block from the MT model which probably represent eclogitised remnants of oceanic lithosphere. Their metamorphosed and exhumed equivalents in association with ultrahigh-temperature metamorphic orogens have been identified from surface geological studies. Both seismic reflections and MT model confirm a southward subduction polarity with a progressive accretion history during the northward migration of the trench prior to the final collisional assembly of the crustal blocks along the Palghat–Cauvery Suture Zone, the trace of the Gondwana suture in southern India.     

Subduction–accretion–collision history along the Gondwana suture in southern India: A laser ablation ICP-MS study of zircon chronology

We report the petrological characteristics and preliminary zircon geochronology based on laser ablation ICP mass spectrometry of the various units in an accretionary belt within the Palghat-Cauvery Shear/Suture Zone in southern India, a trace of the Cambrian Gondwana suture. Zircons extracted from a plagiogranite in association with an ophiolite suite within this suture possess internal structure that suggests magmatic crystallization, and yield mid Neoproterozoic ²⁰⁶Pb/²³⁸U age of 817 ± 16 Ma (error: 1σ) constraining the approximate timing of birth of the Mozambique Ocean floor. Compiled age data on zircons separated from a quartzite and metamorphosed banded iron formation within the accretionary belt yields a younger intercept age of 759 ± 41 Ma (error: 1σ) which we relate to a mid Neoproteozoic magmatic arc. Detrital zircons extracted from the quartzite yield ²⁰⁷Pb/²⁰⁶Pb age peaks of about 1.9–2.6 Ga suggesting that they were sourced from multiple protolithis of Neoarchean and Paleoproterozoic. Metamorphic overgrowths on some zircon grains record ca. 500–550 Ma ages which are in good harmony with the known ages for the timing of high-grade metamorphism in this zone during the final stage of continent collision associated with the birth of the Gondwana supercontinent in the latest Neoproterozoic-Cambrian. The preliminary geochronological results documented in our study correlate with the subduction–accretion–collision history associated with the closure of the Mozambique Ocean and the final amalgamation of the Gondwana supercontinent.     

Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry,zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex

The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541 ± 13 Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461 ± 15 Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715 ± 180 Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related ~ 2530 Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial¹⁷⁶Hf/¹⁷⁷Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.     

Episodic widespread magma underplating beneath the North China Craton in the Phanerozoic: Implications for craton destruction

A comprehensive synthesis of U–Pb geochronology and Hf isotopes of zircons from granulite/pyroxenite xenoliths entrained in Phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the North China Craton (NCC) provides new insights into understanding the Phanerozoic evolution of the lower crust in this craton. Episodic widespread magma underplating into the ancient lower crust during Phanerozoic has been identified throughout the NCC from early Paleozoic to Cenozoic, broadly corresponding to the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan orogenies on the circum-craton mobile belts. The early Paleozoic (410–490 Ma) ages come from xenoliths in the northern and southern margins as well as the central domain of the Eastern Block of the craton which mark the first phase of Phanerozoic magma underplating since the final cratonization of the NCC in the Paleoproterozoic. The magmatism coincided with the northward subduction of the Paleotethysian Ocean in the south and the southward subduction of the Paleoasian Ocean in the north. The subduction not only triggered magma underplating but also led to the emplacement of the diamondiferous kimberlites on the craton, marking the initiation of decratonization. The late Paleozoic event as represented by the 315 Ma garnet pyroxenite and/or lherzolite xenoliths in Hannuoba was restricted to the northern and southern margins of the craton, correlating with the arc magmatism continuous associated with the subduction of the Paleotethysian and Paleoasian Oceans and resulting in the interaction between the melts from subducted slabs and the lithospheric mantle/lower crust. The early Mesozoic event also dominantly occurred in the northern and southern margins and was related with the final closure of the Paleotethysian and Paleoasian Oceans as well as the collisional orogeny between the NCC and the Yangtze Craton. The late Mesozoic (ca. 120 Ma) was a major and widespread magmatic event which manifested throughout the NCC, associated with the geothermal overturn due to the giant south Pacific mantle plume. The Cenozoic magmatism, identified only in the dark clinopyroxenite xenoliths in the Hannuoba, was probably induced by the Himalayan movement in eastern Asia and might also have been influenced by the subduction of the Pacific Ocean to some extent. These widespread and episodic magma underplating or rejuvenation of the ancient lower crust beneath the NCC revealed by U–Pb and Hf isotope data resulted from the corresponding addition of juvenile materials from mantle to lower crust, with a mixing of the old crust with melts. The process inevitably resulted in the compositional modification of the ancient lower crust, similar to the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite — infiltrated melt reaction as revealed in the lithospheric mantle beneath the craton.     

Gemstone Mineralization in the Palghat-Cauvery Shear Zone System (Karur-Kangayam Belt), Southern India

We summarize here the occurrence of a number of semiprecious stones within a major gemstone belt in the Palghat-Cauvery shear zone system close to the northern margin of the Madurai Granulite Block, southern India. The gem mineralization in this belt includes different varieties of corundum (star ruby, sapphire), cordierite (iolite), feldspar (moonstones and sunstone of various hues), beryl (emerald, aquamarine), chrysoberyl (alexandrite), kornerupine, topaz, spinel, crystal quartz and amethyst, among others. Gem mineralisation has not been directly dated, but is associated with pegmatites that crosscut rocks metamorphosed in late Neoproterozoic/Early Cambrian times. Similar aged gem mineralization occurs south of the Achankovil Shear Zone in southern India, as well as in Sri Lanka and Madagascar and strengthen the view of a broad late Neoproterozoic-early Cambrian gemstone province in central Gondwana. The ruby mineralization within this belt can be correlated with similar occurrences in eastern Madagascar, supporting a correlation between the Malagasy Betsimisaraka suture zone and the Palghat-Cauvery shear zone system in southern India.