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[Display omitted] • We report the first reliable age data for Karoo rift magmatism in Mozambique. • Our U/Pb ages prove Karoo rift magmatism started during ca. 185 – 190 Ma. • Eruption of 182–178 Ma rift magmas released abundant volatiles into atmosphere. New U-Pb ages for zircons constrain the duration of silicic magmatism and timing of coeval mafic magmatism across the main rift zone of the Karoo large igneous province in Mozambique. Our 190 ± 2 Ma, 188.4 ± 0.9 Ma, 181.7 ± 1.0 Ma, 180 ± 3 Ma, 178 ± 2 Ma and 172 ± 2 Ma ages support periodicity of Karoo magmatism previously inferred from 40Ar/39Ar age data. The ∼ 190–188 Ma ages confirm early onset of magmatism and the ∼ 182–178 Ma ages correlate the bimodal volcanic successions of the Lower Zambezi and the Movene Formation with widespread silicic magmatism across the rift zone. The ∼ 172 Ma age corresponds to waning magmatic activity. The age range and Hf isotopic compositions of zircons indicate up to ∼9 Ma lifespan for the Jurassic silicic magma chambers and suggest that the ∼2700–400 Ma xenocrysts represent crustal sources of the host rocks. The available chronological data indicate that the ∼183 Ma main phase magmatism was largely confined within the main Karoo and Kalahari basins and that the preceding and subsequent phases were mainly associated with the Karoo rift zone. Judging from geochemical literature, different kinds of magmas were erupted during the successive magmatic phases. We calculate from published geochemical data that the mafic main phase magmas were relatively poor in CO 2 and SO 2 and the lava stacking patterns point to low eruption rates, which suggests that degassing of sedimentary wall-rocks of intrusions probably triggered the coeval Pliensbachian-Toarcian extinction. In contrast, the mafic late phase magmas were rich in CO 2 and SO 2 and at least some of the lavas indicate high eruption rates. We propose that efficient degassing from widespread mafic magmatism and explosive eruption of over 30,000 km3 of silicic magmas in the Karoo rift zone linked the ∼182–178 Ma late phase magmatism with contemporaneous global biosphere crises. [ABSTRACT FROM AUTHOR]

Rubbly pahoehoe lava flows are abundant in many continental flood basalts including the Deccan Traps. However, structures with radial joint columns surrounding cores of flow-top breccia (FTB), reported from some Deccan rubbly pahoehoe flows, are yet unknown from other basaltic provinces. A previous study of these Deccan “breccia-cored columnar rosettes” ruled out explanations such as volcanic vents and lava tubes, and showed that the radial joint columns had grown outwards from cold FTB inclusions incorporated into the hot molten interiors. How the highly vesicular (thus low-density) FTB blocks might have sunk into the flow interiors has remained a puzzle. Here we describe a new example of a Deccan rubbly pahoehoe flow with FTB-cored rosettes, from Elephanta Island in the Mumbai harbor. Noting that (1) thick rubbly pahoehoe flows probably form by rapid inflation (involving many lava injections into a largely molten advancing flow), and (2) such flows are transitional to ‘a’ā flows (which continuously shed their top clinker in front of them as they advance), we propose a model for the FTB-cored rosettes. We suggest that the Deccan flows under study were shedding some of their FTB in front of them as they advanced and, with high-eruption rate lava injection and inflation, frontal breakouts would incorporate this FTB rubble, with thickening of the flow carrying the rubble into the flow interior. This implies that, far from sinking into the molten interior, the FTB blocks may have been rising, until lava supply and inflation stopped, the flow began solidifying, and joint columns developed outward from each cold FTB inclusion as already inferred, forming the FTB-cored rosettes. Those rubbly pahoehoe flows which began recycling most of their FTB became the ‘a’ā flows of the Deccan. [ABSTRACT FROM AUTHOR]

The Qiangtang flood basalt province (QFBP) was recently recognized in northern Tibet, China. It erupted during the Sakmarian–Kungurian and covered an area of > 6.84 × 10 5 km 2 , reaching a maximum thickness of 1.5–2.0 km. The Qiangtang basalts have ocean-island basalt (OIB)-type trace element patterns, with enrichments of light rare-earth elements (LREEs) and Ti–Nb–Ta relative to similarly incompatible elements, as well as exhibiting no Eu-anomalies. The basalts show medium to high Sr and Pb isotopic ratios, and consistently positive ε Nd (t). Strong correlations between Mg index, and major and trace elements indicate that the basalts have experienced fractional crystallization of clinopyroxene and olivine, and may have been generated by partial melting of an OIB-type mantle plume source with negligible crustal contamination. Similar early Permian basalts are also present in the Lhasa terrane, South Tibet–northern India, the Sanjiang area, and the Tarim basin. These basalts have an estimated original volume of > 7.51 × 10 5 km 3 , spread over an area of > 9.84 × 10 5 km 2 . We propose that the large-scale eruption of these flood basalts was related to a Sakmarian–Kungurian mantle plume that possibly initiated the disintegration of northern Gondwana and the formation of the Tethys oceans. [ABSTRACT FROM AUTHOR]

Abstract: A set of whole rock lava flows and dykes from the western part of Deccan Traps, Western India, was analyzed for oxygen isotope ratios. δ18O stratigraphy of the upper formations in the Mahabaleshwar section of the Western Ghats area can be correlated with the Sr- and Nd-isotopic stratigraphy, in that increasing δ18O is associated with increasing initial 87Sr/86Sr and decreasing initial ɛNd. The increases of δ18O (up to +11.8‰) exceed the range of primitive mantle-derived materials, strongly suggesting that isotopic variations of these Deccan basalts are due to crustal contamination rather than input of continental lithospheric mantle as previously considered. The variations of δ18O–87Sr/86Sr and δ18O–ɛNd indicate that there are at least two contaminating domains, most likely (1) Archean granitic upper crust and (2) Archean carbonates+granulitic lower crust. [Copyright &y& Elsevier]

Abstract: We discuss the results of a field experiment in the Malaya Botuobiya area (West Yakutia) at a site where earlier surveys revealed slowly decaying transient responses. That time-dependent voltage decay indicated magnetic viscosity effects associated with magnetic relaxation of superparamagnetic grains in rocks. In this study, we have applied a high-resolution array TEM survey to contour the anomaly and parametric soundings with systems of different configurations to explore the vertical pattern of magnetic viscosity. The parametric data have been inverted, by means of manual and automated fitting, with a reference model of a layered magnetically viscous earth, using, respectively, analytical formulas and simulation based on a forward solution by separation of variables. According to both automated and manual inversion, the section at the center of the anomalous site fits a three-layer earth model with an intermediate magnetically viscous layer between two nonmagnetic layers. This model is consistent with a priori evidence of local geology and may provide more details of the latter. The inversion results have been further used to estimate the volumetric percentage of superparamagnetic grains in the magnetically viscous layer, assuming magnetite to be the main ferrimagnetic phase. [ABSTRACT FROM AUTHOR]