粤西阳春中生代钾玄质侵入岩及其构造意义:Ⅱ .微量元素和Sr- Nd同位素地球化学

Mesozoic shoshonitic intrusives in the Yangchun Basin, western Guangdong, and their tectonic significance:Ⅱ . Trace elements and Sr- Nd isotopes

粤西阳春地区马山二长闪长岩强烈富集 K、 Sr和 LREE， (87Sr/86Sr)i≈ 0.704 6，ε Nd(t)≈＋ 1；岗尾-轮水岩体较富集 K、 Rb、 Th和 LREE， (87Sr/86Sr)i≈ 0.706 3，ε Nd(t)≈－ 2；石岩体较富集 Sr， K、 Rb、 Th和 LREE相对较低， (87Sr/86Sr)i=0.708 4～ 0.708 9，ε Nd(t)≈－ 6。马山岩体来源于大离子亲石元素 (LILE)和 LREE富集的交代地幔；岗尾-轮水岩体来自于放射成因 Sr、 Nd同位素组成略高或交代时间略早的富集交代地幔，并且经历了明显的结晶分异作用；石岩体则很可能是前存下地壳底垫基性岩重熔形成的。从早侏罗世到早白垩世，南岭西部的岩浆成分和源区的规律性变化反映了区域软流圈地幔上涌和岩石圈伸展-拉张-减薄的演化过程。

Trace elements and Sr- Nd isotopic data are presented for the Mesozoic intrusives in the Yangchun Basin, western Guangdong. The Mashan monzo- diorites are characterized by strong enrichment of K, Sr and LREE, and nearly chondritic isotopic compositions with (87Sr/86Sr)i≈ 0.704 6 andε Nd(t)≈ 1. The Gangwei- Lunshui quartz monzonites and granodiorites are moderately enriched in K, Sr, Rb and LREE, but strongly enriched in Th. They have enriched Sr- Nd isotopic compositions with (87Sr/86Sr)i≈ 0.706 3 andε Nd(t)≈－ 2. The Shilu granodiorites are enriched in Sr, but less enriched in K, Rb, Th and LREE. They are characterized by more radiogenic Sr- Nd isotopic compositions with (87Sr/86Sr)i = 0.708 4～ 0.708 9 andε Nd(t)≈－ 6. It is concluded, in terms of geochemical and Sr- Nd isotopic features, that the Mashan monzo- diorites were generated from a metasomatism mantle source strongly enriched in LILE and LREE, while the Gangwei- Lunshui plutons were derived from a metasomatism mantle source with more radiogenic Sr- Nd isotopic compositions followed by extensive crystal fractionation. The Shilu pluton was formed by re- melting of pre- existing underplating mafic rocks at the lower crustal level. The secular variation of magma compositions was mostly attributed to the upwelling of asthenosphere and extension of overlying lithosphere in the Nanling Region during the early Jurassic to early Cretaceous.