Geochemistry of the Picrites and Associated Basalts from the Emeishan Large Igneous Basalt Province and Constraints on Their Source Region
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Abstract:
The geochemical characteristics of the picritic rocks that we discovered recently and their associated basalts show that except the picrites, all the associated basaltic rocks belong to the high-Ti type. Like many other continental flood basaltic provinces, they are characterized by high Fe8 and (CaO/Al2O3)8 and low Na8, indicating high pressure. Their chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns are similar, all of which display an enrichment of LREE and relative depletion of high field strength elements (HFSE) associated with an absence of Nb and Ta negative anomalies but a presence of P and K negative anomalies, as characterized by most flood basalts. Some trace element ratios characterizing the source regions, such as La/Ta, La/Sm, (La/Nb)pM, (Th/Ta)PM, Ta/Hf and Nb/Zr are within a narrow range. All these observations suggest the origin of mantle plume, and no or little crustal or lithospheric mantle contamination. They are generated by about 7% of partial melting of garnet Iherzolite at more than 75 km. Thus, the axis of the Emeishan mantle plume should be located beneath the Lijiang County Town, Yunnan Province.Keywords:
Flood basalt
Large igneous province
Mantle plume
Trace element
Hotspot (geology)
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Major and trace elements analysis has been carried out on the Late Ladinian Tabai basalts from Yunnan Province with the aim of studying their petrogenesis. Their SiO2 contents range from 43.63 wt.% to 48.23 wt.%. The basalts belong to the weakly alkaline(average total alkalis Na2 O+K2O=3.59 wt.%), high-Ti(3.21 wt.% to 4.32 wt.%) magma series. The basalts are characterized by OIB-like trace elements patterns, which are enriched in large ion lithosphile elements(LILE) including Rb and Ba, and display negative K, Zr and Hf anomalies as shown on the spider diagrams. The Tabai basalts display light rare-earth elements(LREE) enrichment and are depleted in heavy rare-earth elements(HREE) on the REE pattern. Those dates indicate that the parental magma of the Tabai basalts was derived from low-degree(1%–5%) partial melting of garnet peridotite. The magma underwent olivine fractional crystallization and minor crustal contamination during their ascent. The Tabai basalts were related to a relaxation event which had triggered the Emeishan fossil plume head re-melting in the Middle Triassic.
Petrogenesis
Fractional crystallization (geology)
Ladinian
Lile
Peridotite
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Phenocryst
Large igneous province
Flood basalt
Fractional crystallization (geology)
Melt inclusions
Mantle plume
Igneous differentiation
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We report concentrations of platinum group elements (PGE: Os, Ir, Ru, Rh, Pt and Pd) for picrite-basalt lavas of the Emeishan large igneous province, SW China. The absolute concentrations of Os in most of the Emeishan picrites are higher than in picrites from ocean islands and other continental flood basalt provinces, and in estimated primitive mantle. A roughly positive correlation between MgO and Cr and Ni, and no overall correlation of Os or Ir with MgO, show that no Cu-Ni sulfide fractionated during differentiation but rather that olivine and chromite fractionated, implying that the initial picritic magma was sulfide-undersaturated. In addition, Os/Ir ratios are unusually high, suggesting that the picritic magmas may have been contaminated by black shales. Modeling shows that a 7% partial melt of primitive mantle containing 0.01 wt% of sulfides can account for the observed Os, Ir and Ru concentrations in the most primitive picrite if the melt is contaminated by 10 wt% of black shale en route to the surface.
Large igneous province
Flood basalt
Platinum group
Chromite
Mantle plume
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Silicic
Large igneous province
Felsic
Mantle plume
Flood basalt
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Phenocryst
Porphyritic
Petrogenesis
Island arc
Chromite
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The late Cretaceous-early Tertiary flood basalts in the Gujarat area of the northwestern Deccan Traps (Kathiawar peninsula, Pavagadh hills and Rajpipla) exhibit a wide range of compositions, from picrite basalts to rhyolites; moreover, the basaltic rocks have clearly distinct TiO2 contents at any given degree of differentiation and strongly resemble the low-titanium and hightitanium basalts found in most of the Gondwana continental flood basalt (CFB) suites. Four magma groups are petrologically and geochemically distinguished: (1) A low-Ti group, characterized by rocks with varying SiO2 saturation, and with TiO2 <1⋅8 wt%, extremely low incompatible trace element abundances, low Zr/γ (av- 3⋅8), Ti/ V (av. 27), and a very slight large ion lithophile element (LJLE) enrichment over high field strength elements (HFSE). These rocks share some features with the Bushe Formation of the Western Ghats farther south, but have distinct geochemical characters, in particular the strong depletion in most incompatible trace elements. (2) A high-Ti group, characterized by a more K-rich character than the low-Ti rocks, and with a strong enrichment in incompatible elements, similar to average ocean island basalt (OIB), e.g. high TiO2 (>1⋅8 wt% in picrites), Nb (>19 p.p.m.) Zr/γ (av. 6⋅5) and Tt/V (av. 47). (3) An intermediate-Ti group, with TiO2 contents slightly lower than the high-Ti rocks at the same degree of evolution, and with correspondingly lower incompatible trace element contents and ratios, in particular K2O, Nb, Ba and Zr/Y (av. 5⋅2). (4) A potassium-rich group (KT), broadly similar in geochemical character to the high-Ti group but showing more extreme K, Rb and Ba enrichment (av. K20/Na20∼l; Ba/Y∼20). The most primitive low-Ti and high-Ti picrites, when corrected for low-pressure olivine fractionation, show distinct major (and trace) element geochemistry, in particular for CaO/AI2O3, CaO/TiO2 and Al2O3/TiO2, and moderate but significant variations in their SiO2 and Fe2Ost contents; these characteristics strongly suggest the involvement of different mantle sources, more depleted for the low-Ti picrites, and richer in cpxfor the high-Ti picrites, but with broadly the same pressures of equilibration (27–14 kbar). This, in turn, suggests a strong lateral heterogeneity in the Gujarat Trap mantle. Low-Ti picrites and related differentiates in Kathiawar are reported systematically for the first time here, and suggest the existence of HFSE-depleted mantle in the northwestern Deccan Traps, with extension at least to the Seychelles Islands and to the area of the Bushe Formation near Bombay in the pre-drift position, before the development of the Carlsberg Ridge. The absence of correlations between LILE/HFSE ratios and SiO2 argues against crustal contamination processes acting on the low-Ti picrites, possibly owing to their probably rapid uprise to the surface. Consequently, the mantle region of this rock group was probably re-enriched by small amounts of ULE-rich materials. The substantially higher, trace element enrichment of the least differentiated high-Ti picrites, relative to the basalts of the Ambe-noli and Mahableshwar Formations of the Western Ghats, testifies also to the presence of more incompatible element rich, OIB4ike mantle sources in northern and northwestern Gujarat. These sources were geochemicaily similar to the present-day Reunion mantle sources.
Flood basalt
Lithophile
Deccan Traps
Incompatible element
Trace element
Petrogenesis
Lile
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