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The Mundwara alkaline plutonic complex (Rajasthan, north-western India) is considered a part of the Late Cretaceous-Palaeogene Deccan Traps flood basalt province, based on geochronological data (mainly Ar/Ar, on whole rocks, biotite and hornblende). We have studied the petrology and mineral chemistry of some Mundwara mafic rocks containing mica and amphibole. Geothermobarometry indicates emplacement of the complex at middle to upper crustal levels. We have obtained new Ar/Ar ages of 80-84 Ma on biotite separates from mafic rocks and 102-110 Ma on whole-rock nepheline syenites. There is no evidence for excess Ar. The combined results show that some of the constituent intrusions of the Mundwara complex are of Deccan age, but others are older and unrelated to the Deccan Traps. The Mundwara alkaline complex is thus polychronous and similar to many alkaline complexes around the world that show recurrent magmatism, sometimes over hundreds of millions of years. The primary biotite and amphibole in Mundwara mafic rocks indicate hydrous parental magmas, derived from hydrated mantle peridotite at relatively low temperatures, thus ruling out a mantle plume. This hydration and metasomatism of the Rajasthan lithospheric mantle may have occurred during Jurassic subduction under Gondwanaland, or Precambrian subduction events. Low-degree decompression melting of this old, enriched lithospheric mantle, due to periodic diffuse lithospheric extension, gradually built the Mundwara complex from the Early Cretaceous to Palaeogene time. [ABSTRACT FROM AUTHOR]

The Sindreth Group exposed near Sirohi in southern Rajasthan, western India, is a volcanosedimentary sequence. Zircons from Sindreth rhyolite lavas and tuffs have yielded U-Pb crystallization ages of ~768-761 Ma, suggesting that the Sindreth Group is a part of the Malani magmatic event. Earlier Ar-Ar studies of other Malani volcanic and plutonic rocks yielded disturbed argon release spectra, ascribed to a ~550 Ma thermal event possibly related to the Pan-African orogeny. To test and confirm this possibility, we dated two whole-rock and three feldspar separate samples of the Sindreth volcanics by the Ar-Ar method. All samples yield disturbed argon release spectra suggesting radiogenic argon loss and with plateau segments at 550 Ma or 490 Ma. We interpret these as events of argon loss at 550-490 Ma related to an Ediacaran-Cambrian thermal event, possibly related to the Malagasy orogeny. The combined older and new Ar-Ar results are significant in showing that whereas Ediacaran-Cambrian magmatic and metamorphic events are well known from many parts of India, they left thermal imprints in much of Trans-Aravalli Rajasthan as well. The overall evidence is consistent with a model of multiphase assembly of Gondwanaland from separate continental landmasses. [ABSTRACT FROM AUTHOR]

This study provides further evidence for a Cryogenian (∼770–760 Ma) deformation event in NW India, a region generally regarded tectonically stable during the Rodinia to Gondwana transition. This deformation is prominent in the Sirohi region (southwestern Rajasthan State), previously referred to as the ‘Sirohi Orogeny’, younger to and independent of the ∼1 Ga to 820 Ma Delhi Orogeny. Synkinematic intrusion of the Balda Granite during the NW-directed thrusting and folding of sediments of the Neoproterozoic Sirohi Group is constrained by combined field, microstructural and magnetic fabric analyses. Thrusting (D 1 ) occurred under medium grade deformation conditions along mylonitic to ultramylonitic shear zones featuring quartz and feldspar recrystallization. Steepening of thrust planes and transition from dip-slip kinematics to oblique and dextral strike-slip in high strain areas indicate transpressive kinematics during the late-stage of D 1 deformation. A second deformation event (D 2 ) is documented in strain concentration along a conjugate set (N–S & E–W) of shear zones wherein dilation along the N–S trending structures provided space for voluminous synkinematic quartz mineralization. This linear structure, traceable for ∼10 km and named ‘Balda-Paladi Shear Zone’, has overprinted the NE–SW trending magmatic fabrics in the Balda Granite by N–S oriented shearing. Brittle behavior of feldspar constrains this overprint as a low temperature deformation event that also represents the terminal Cryogenian imprint in the Sirohi sector. The findings of this study indicate that an early Cryogenian imprint, seen in three different tectonomagmatic zones, from E to W referred to as South Delhi high-grade metamorphic terrane, Sirohi anatectic terrane and Sirohi fold and thrust terrane, is coeval with the initial closure of the Mozambique Ocean as constrained for central and northern Madagascar. A continuation of early Cryogenian mobile belts extending from southern India, crossing Madagascar into NW India is proposed. Related structures could have served as pathways for heat convection and triggered local late Pan-African thermal resetting in NW India. [ABSTRACT FROM AUTHOR]

Abstract: The Tavidar volcanic suite in western Rajasthan, India, comprises a group of lava flows (and subordinate pyroclastic deposits) of highly diverse compositions ranging from basalt through trachyte to rhyolite. We have dated five samples of the Tavidar volcanic rocks by the 40Ar–39Ar incremental heating technique. One trachyte and two rhyolite samples yield very good plateau, isochron and inverse isochron ages of 67–65Ma, typical of the Deccan Traps large igneous province. A subalkalic basalt and a basaltic trachyandesite yield saddle-shaped argon release spectra and show evidence for excess argon. Importantly, all five samples have very similar Nd–Sr isotopic ratios, and constitute a broadly cogenetic magmatic suite with the rhyolites possibly derived by closed-system fractional crystallization of trachytic magmas. The Tavidar rocks’ isotopic data overlap, or are very close to, those of the Mahabaleshwar and Panhala Formation basalts in the Western Ghats type section 700–800km to the southeast. We therefore infer that the Tavidar rocks, having initial Nd values of +3.2 to +0.7, have incorporated only small amounts of lower continental crust. The Tavidar volcanic suite attests to the great areal extent of the Deccan Traps, and reaffirms the great compositional diversity evident in the northwestern Deccan Traps. [Copyright &y& Elsevier]

• Pur-Banera, a supracrustal sequence near Bhilwara, Aravalli Craton. • Structural mapping and zircon–monazite geochronology. • Complementary age records from zircons and monazites. • Magmatism at 3.2, 2.5, 1.8 Ga; Sedimentation at 1.6–1.3 Ga; Deformation at 1.3, 1.05, 0.85 Ga. • Relationship to assembly and breakup of supercontinent Rodinia. The Banded Gneissic Complex (BGC) of the Aravalli Craton in Rajasthan, northwestern India, forms the Archaean basement to the major, extensively studied, Proterozoic-age metasedimentary sequences of the Aravalli and Delhi Supergroups. However, the poorly documented deformation history of the basement and other supracrustal sequences, and insufficient geochronological work, limit a holistic understanding of the evolution of the Aravalli Craton. Here we present structural analysis and zircon and monazite geochronology of the relatively poorly studied Pur-Banera supracrustal belt in the Bhilwara region of south-central Rajasthan. The belt comprises paragneisses, carbonates, calc-gneisses, banded iron formations, garnetiferous schists, and amphibolites, deposited over Aravalli Supergroup rocks and locally also granite. A single detrital zircon core from the Pur-Banera paragneisses yields a 207Pb/206Pb age of 3159 ± 8 Ma (1σ), indicating a Mesoarchaean protolith. Magmatic zircon grains from the granite, here termed the Malola Granite, provide a 207Pb/206Pb crystallisation age of 2538 ± 11 Ma (95% confidence). The Pur-Banera supracrustals have experienced three phases of deformation, D 1 –D 3 , each of which produced a tectonic fabric. Zircon and monazite geochronology provides complementary ages. The detrital zircons, derived from both magmatic and metamorphic protoliths, provide ages of 3.2–1.7 Ga. Monazite ages of 1.3–0.8 Ga record multiple post-depositional tectonothermal events. Textural and compositional evidence and U–Th–total Pb monazite ages indicate that D 1 deformation occurred at ca. 1.30 Ga as a result of NW–SE-directed shortening, forming upright folds on the primary foliation that resulted in a penetrative fabric in the gneisses and schists. Subsequent NNE–SSW-directed deformation (D 2) between 1.13 and 1.05 Ga produced WNW–ESE-trending vertical folds in the gneisses and crenulations in the schists. Asymmetrical folds developed due to N–S dextral shearing at 0.85–0.80 Ga constitute the last phase of deformation (D 3). The Pur-Banera supracrustal rocks probably represent a rift-fill sequence, of Mesoproterozoic age (thus temporally equivalent to the Delhi Supergroup), deposited on an Archaean and Palaeoproterozoic basement, and repeatedly reworked during 1.30–0.80 Ga. [ABSTRACT FROM AUTHOR]

The Banded Gneissic Complex (BGC) of the Aravalli Craton (Rajasthan, northwestern India) is considered to have evolved through the amalgamation of several crustal terranes. The BGC has been previously divided into an older, Archaean, BGC-I (exposed south of Nathdwara and roughly equivalent to Mewar Gneiss) and a younger, Proterozoic, BGC-II (exposed north of Nathdwara). However, the poorly documented deformation history and insufficient geochronological work limit our understanding of BGC evolution. To understand this, we combine detailed structural analysis of basement gneisses (traditionally considered to be BGC-II) and a metasedimentary cover sequence (the Pur-Banera belt) exposed in the Bhilwara region, with U–Pb zircon and U–Th–total Pb monazite geochronology. Resorbed zircon cores from the basement gneisses yield a 207Pb–206Pb age of 3159 ± 8 Ma (1σ). This oldest analysed population, giving a minimum crystallization age, is hitherto unreported from BGC-II. The basement gneisses experienced four phases of deformation, D1 through D4, each of which developed a tectonic fabric. Textural evidence and U–Pb zircon ages indicate the D1 deformation to be 2.9 Ga in age. A NNW–SSE-directed shortening event (D2) at 2.5 Ga formed upright folds on early gneissic foliation (S0//S1), with syn-tectonic granite emplacement. Subsequent NNE–SSW-directed deformation (D3) at 1.8 Ga produced NW–SE-trending vertical folds. Asymmetrical folds developed due to N–S-trending dextral shearing at 1.3 Ga constitute the last phase of deformation (D4) in the basement gneisses. The Pur-Banera sequence shows NNE–SSW-directed shortening on primary foliation, forming S1 foliation along WNW–ESE direction. These metapelitic supracrustals also show tight crenulations on S1 foliation, with pervasive axial planar fabric (S2), typically observed in high-strain zones formed at 1.3 Ga. This suggests that the two deformation events experienced by the Pur-Banera sequence correspond to the D3 and D4 deformation events affecting the basement gneisses. The ubiquitous record of older age populations (3.2–1.3 Ga) in zircon grains, with younger age populations (1.3–0.8 Ga) mostly preserved in monazite grains, indicates that monazite growth was associated with younger tectonic events that reworked large parts of the basement gneisses and supracrustal rocks. Our results demonstrate that BGC-I and BGC-II, with Mesoarchaean ages and known similar geochemical signatures, are not different. The BGC thus represents a coherent crustal entity formed during the Archaean and reworked several times during the Proterozoic.Keywords: Aravalli Craton, Rajasthan, Banded Gneissic Complex, Geochronology, Crustal evolution [ABSTRACT FROM AUTHOR]