会昌早白垩世橄榄玄粗岩(shoshonite)成因的元素及Sr-O-Nd-Pb同位素地球化学证据

Geochemical evidence of element and Sr-O-Nd-Pb isotopes for petrogenesis of the Huichang Early Cretaceous shoshonite,southern Jiangxi province

根据赣南会昌地区中基性火山岩高碱、富钾、低钛、贫铁,岩石中斜长石斑晶具钾长石环边,基质中存在大量钾长石微晶,以及富集轻稀土元素和大离子亲石元素等矿物-岩石地球化学特征,确切厘定会昌地区的中-基性火山岩为橄榄玄粗岩-安粗岩组合,属典型的大陆板内橄榄玄粗岩系列火山岩。对会昌橄榄玄粗岩进行了Rb-Sr同位素定年研究,确定其全岩Rb-Sr等时线年龄为107.3±2.3Ma。会昌橄榄玄粗岩系火山岩Sr-O-Nd-Pb同位素组成的特征为:偏高的ISr(0.7098~0.7115);较低的δ18O值(5.3‰~7.0‰);中等的εNd(t)(-0.61^-3.60);富放射性成因铅((206Pb/204Pb)i=17.32~18.29,(207Pb/204Pb)i=15.34~15.65,(208Pb/204Pb)i=37.51~38.60)。会昌橄榄玄粗岩的Δ7/4Pb值为-7.8^+16.8(平均值为+5.42),Δ8/4Pb值为27.3~97.3(平均值为68.04),ΔSr值为96.2~114.1(平均值为104.3),这表明存在典型的Dupal同位素异常。根据Sr-O,Sr-Nd,Sr-Pb,Nd-Pb,Pb-Pb同位素相关特征,判明会昌橄榄玄粗岩是由亏损地幔端元(DM)和岩石圈富集地幔端元(EM)在源区混合形成的。按Sr-Nd双变量二元混合模型计算得出源区物质中亏损地幔端元和富集地幔端元所占份额各占50%左右。会昌早白垩世橄榄玄粗岩系火山岩带的形成反映了华南板块内部在燕山晚期发生的一起重要的伸展构造事件。

The intermediate basic volcanic rocks in the Huichang area of southern Jiangxi province are characterized by ： high total alkalis, enrichment of K2O, low TiO2 content, low iron content and the enrichment of LREE and LILE elements. In combination with the mineralogical features such as the appearance of orthoclase rims along the plagioclase boundary and the existence of orthoclase micro-crystals in the groundmass, it is identified to be shoshonite-latite association and belong to typical interplate shoshonitic volcanic rock series. The Rb-Sr isochron age of the Huichang shoshonite series （HSS）is 107.3 ±2.3Ma. The Sr-O-Nd-Pb isotope compositions of HSS are characterized by the follows： distinctive high initial Sr isotopic ratios （Ist=0. 7098-0. 7115）： lowδ^18O value （5.3‰-7.0‰）; intermediate initial Nd isotopic rat ious （εNd （t） = -0.61 - 3.60） ; enrichment of radiogenic lead （（^206 pb/^204 Pb）i= 17.32‰ 18.29‰, （^207Pb/^204Pb）i=15.34-15.65, （^208Pb/^204Pb）i=37.51-38.60）. The HSS have obvious Dupal anomaly（△7/4Pb=-7.8-＋16.8,averagely ＋5.42） ; △8/4Pb=27. 3-97. 3 （averagely 68.04） ; △Sr= 96.2-114. 1 （averagely 104.3））. The Sr-O, Sr-Nd, Sr-Pb, Pb-Pb relationships of HSS strongly suggest a mixing source of the depleted mantle endmember with the lithosphere enriched mantle endmember. According to double-variation and two-component （Sr-Nd） mixing equation, calculation shows that the depleted mantle endmember and the lithosphere enriched mantle endmember are about 50% for rockforming materials of HSS. The exisitence of the Huichang Early Cretaceous shoshonitic volcanic rock belt implies a major extensional tectonic event during the Late Yianshanian period.