Johannesburg, South Africa
Johannesburg, South Africa

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Medlin C.C.,Monash University | Medlin C.C.,De Beers Group Exploration | Jowitt S.M.,Monash University | Cas R.A.F.,Monash University | And 6 more authors.
Journal of Petrology | Year: 2014

The Pussy Cat Group rhyolites of the Mesoproterozoic west Musgrave Province of central Australia, a constituent part of the Bentley Supergroup, were deposited during the c. 1085-1040Ma Ngaanyatjarra Rift and Giles events, and are related to the Warakurna Large Igneous Province. This study focuses on the two silicic components of the Pussy Cat Group, the Kathleen Ignimbrite and the Rowland Suite. These silicic rocks are A-type, metaluminous (to slightly peraluminous) rhyolites and are enriched in the rare earth elements (REE) relative to average crustal abundances. The rhyolitic Kathleen Ignimbrite records an explosive caldera fill-sequence and contains, amongst others, a thick (≤500 m), initially subaqueously emplaced, rheomorphic, intra-caldera ignimbrite unit, whereas the Rowland Suite consists of a number of mineralogically and geochemically related porphyritic rhyolites that intrude throughout the Pussy Cat Group. Whole-rock geochemistry, Rb-Sr, Sm-Nd and in situ zircon Lu-Hf isotope data are indicative of a dominantly mantlederived source for the magmas that formed the Pussy Cat Group rhyolites. Secondary ion mass spectrometry U-Pb dating of these units yields ages of 1062±8, 1071±5, 1076±5, and 1078±5 Ma. The magmas that formed these units were formed by extreme fractional crystallization of a mantle-derived basaltic magma, with minimal crustal contamination, during a failed intraplate extensional rifting event. This involved three main stages of fractional crystallization: early fractionation of plagioclase, olivine, clinopyroxene and magnetite from a basaltic magma to reach an intermediate composition, subsequent fractionation of plagioclase, K-feldspar and quartz to form a proto-Rowland Suite-type magma at mid- to upper-crustal levels that migrated into the shallow upper crust and formed a magma chamber, and final fractionation of quartz, K-feldspar, plagioclase, magnetite and biotite±minor REE-enriched accessory phases from the Rowland Suite magma resulting in the evolved Kathleen Ignimbrite magmas. This final phase of fractionation generated the most evolved silicic rock suite identified to date within the entire west Musgrave Province. The new petrographic, geochronological, geochemical, and isotopic data presented within this study indicate that these two units are coeval and comagmatic, suggesting a common source for the Kathleen Ignimbrite and the entire Rowland Suite. In addition, these data suggest that the crystal-rich, porphyritic rhyolite intrusions of the Rowland Suite represent a primitive cumulate end-member of the magmatic system, whereas the varying crystal-poor to crystal-rich Kathleen Ignimbrite eruption sequence represents the evolved and highly fractionated endmember of the system that formed thorough the evacuation of a shared or at least partly linked, compositionally zoned and differentiated source magma chamber or chambers. © The Author 2015.

Sun J.,CAS Institute of Geology and Geophysics | Sun J.,University of Chinese Academy of Sciences | Liu C.-Z.,CAS Institute of Geology and Geophysics | Tappe S.,De Beers Group Exploration | And 6 more authors.
Earth and Planetary Science Letters | Year: 2014

We report combined U-Pb ages and Sr-Nd isotope compositions of perovskites from 50 kimberlite occurrences, sampled from 9 fields across the Yakutian kimberlite province on the Siberian craton. The new U-Pb ages, together with previously reported geochronological constraints, suggest that kimberlite magmas formed repeatedly during at least 4 episodes: Late Silurian-Early Devonian (419-410 Ma), Late Devonian-Early Carboniferous (376-347 Ma), Late Triassic (231-215 Ma), and Middle/Late Jurassic (171-156 Ma). Recurrent kimberlite melt production beneath the Siberian craton - before and after flood basalt volcanism at 250 Ma - provides a unique opportunity to test existing models for the origin of global kimberlite magmatism. The internally consistent Sr and Nd isotope dataset for perovskites reveals that the Paleozoic and Mesozoic kimberlites of Yakutia have distinctly different initial radiogenic isotope compositions. There exists a notable increase in the initial 143Nd/144Nd ratios through time, with an apparent isotopic evolution that is intermediate between that of Bulk Earth and Depleted MORB Mantle. While the Paleozoic samples range between initial 87Sr/86Sr of 0.7028-0.7034 and 143Nd/144Nd of 0.51229-0.51241, the Mesozoic samples show values between 0.7032-0.7038 and 0.51245-0.51271, respectively. Importantly, perovskites from all studied Yakutian kimberlite fields and age groups have moderately depleted initial εNd values that fall within a relatively narrow range between +1.8 and +5.5. The perovskite isotope systematics of the Yakutian kimberlites are interpreted to reflect magma derivation from the convecting upper mantle, which appears to have a record of continuous melt depletion and crustal recycling throughout the Phanerozoic. The analyzed perovskites neither record highly depleted nor highly enriched isotopic components, which had been previously identified in likely plume-related Siberian Trap basalts. The Siberian craton has frequently been suggested to represent a type example of an association between kimberlite eruptions and flood basaltic volcanism within a single large igneous province (LIP), but our new extensive age and isotopic tracer constraints do not support a genetic link between these contrasting types of mantle-derived magmatism. © 2014 Elsevier B.V.

Joy S.,De Beers Group Exploration | Jelsma H.A.,De Beers Group Exploration | Preston R.F.,De Beers Group Exploration | Kota S.,De Beers India Exploration
Geological Society Special Publication | Year: 2012

The Banganapalle Quartzite Formation occurs in the Cuddapah Basin in India and is characterized by a basal diamond-bearing conglomerate horizon. The diamonds within this placer are generally thought to have been sourced from the erosion of kimberlites of the Wajrakurur cluster. De Beers' India's exploration efforts have resulted in the discovery of a number of dykes within the basin, with petrographical and geochemical similarities to lamproites. It is proposed that far-field stresses related to the Eastern Ghats Mobile Belt (EGMB) provided extensional sites during the time of lamproite emplacement. The dykes have not been dated. However, zircons recovered from heavy mineral stream samples in the area exhibit a number of age groupings, including one in the range of 1287-1370 Ma. This age is interpreted as the emplacement age of the dykes in this region. Kimberlitic indicator minerals (KIMs), recovered from conglomerate waste dumps, indicate the uniqueness of the garnet population relative to that of the known kimberlite clusters to the west of the basin. We propose that the emplacement of lamproites occurred as dyke-sill complexes at 1.4-1.3 Ga and that the lamproites represent the source of the diamonds in the Banganapalle conglomerates.

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