中国东部中生代高Sr低Yb和低Sr高Yb型花岗岩:对比及其地质意义

Mesozoic high-Sr and Low-Yb granitoids and low-Sr and high-Yb granitoids in eastern China:comparison and geological implications.

中国东部在晚中生代时(晚侏罗世-早白垩世)有广泛的中酸性岩浆活动,按照花岗岩的地球化学特征,大致可以划分为东北、华北和华南3个岩区。本文研究表明,按照 Sr 和 Yb 的含量,大致可以将花岗岩分为5类,即:高 Sr 低 Yb 型(Sr >400μg/g,Yb<2μg/g)、低 Sr 低 Yb(Sr<400μg/g,Yb<2μg/g)、低 Sr 高 Yb(Sr<400μg/g,Yh>2μg/g)、高 Sr 高 Yb 型(Sr>400μg/g,Yb>2μg/g)以及非常低 Sr 高 Yb 型(Sr<100μg/g,Yb=2-18μg/g)花岗岩。东北和华南以发育低 Sr 高 Yb 花岗岩为主,有少量高 Sr 低 Yb 和非常低 Sr 高 Yb 类型的花岗岩分布;而华北则以高 Sr 低 Yb 型花岗岩(埃达克岩)最发育,低 Sr 高Yb、低 Sr 低 Yb 型和非常低 Sr 高 Yb 型花岗岩有少量分布。本文着重探讨了华北和华南花岗岩的特征,认为华北和华南花岗岩地球化学的区别可能主要与花岗岩源区成分和深度有关,且主要受源区深度的控制。如果花岗岩熔融的源区残留相由榴辉岩组成(石榴石+辉石+金红石+/-角闪石),则花岗岩明显亏损 HREE、Nb、Ta 和 Ti,而富集 Sr 和 Al,无明显的负铕异常,属于高 Sr 低 Yb(埃达克岩)类型;如果源区深度浅,由斜长角闪岩或麻粒岩组成(斜长石+辉石+角闪石),则花岗岩相对贫Sr 富 Yb。作者认为,华北和华南花岗岩地球化学特征上的上述差异,表明在晚中生代时(晚侏罗世-早白垩世),华北和华南的地壳厚度不同:华北较厚,华南较薄;华北经历了下地壳拆沉而华南无;华北和华南的下地壳成分不,华北较基性的下地壳拆沉后,留下的地壳平均成分与华南比偏中性。

Late Mesozoic intermediate-acid granitoids are widespread in eastern China. They are basically distributed in three regions according to the geochemical characteristics： Northeastern China, North China and Southeastern China. Theoretically granitoids can be divided into five types based on their Sr and Yb contents： i. e. , high-Sr and low-Yb type （Sr 〉400μg/g, Yb 〈 2μg/g）, low- Sr and low-Yb type （Sr 〈400μg/g, Yb 〈 2μg/g）, low-Sr and high-Yb type （Sr 〈400μg/g, Yb 〉 21μg/g） , high-St and high-Yb type （Sr 〉400μg/g, Yb 〉 2μg/g）, and very low-Sr and very high-Yb type （Sr 〈 100μg/g, Yb = 2 - 18μg/g）. The granitoids in NE China and SE China are mainly composed of low-Sr and high-Yb type with minor high-Sr and low-Yb type and very low-Sr and very high-Yb type, whereas the granitoids in North China consist mainly of high-Sr and low-Yb type with minor low-Sr and high-Yb type and low-Sr and low-Yb type and very low-Sr and very high-Yb type. Based on the geochemical characteristics of these granitoids, it is suggested here that the geochemical diversity of the granitoids in the North China and in the SE China is mainly caused by the composition and depth of the magma source and have a close correlation with the depth of the source. High-Sr and low-Yb type granitoids are derived from a magma source with eclogite composed of garnet ＋ pyroxene ＋ rutile ± amphibole as a residual phase such that the evolved granitoids are highly depleted in HREE, Nb, Ta and Ti and enriched in Sr and Al without negative Eu anomalies, consistent with the characteristics of adakite. Low-Sr and high-Yb type granitoids are formed in a relatively shallow magma source where amphiolites or gneisses composed of plagioclase ＋ pyroxene _ amphiolite are the major residual phases. These differences indicate that the crust thickness of the North China and SE China is different in the late Mesozoic （ late Jurassic to early Cretaceous） ： the crust of the North China was thicker than that of the SE China. North China may have undergone delamination such that the chemical composition of the lower crust of the North China is different from that of the SE China： the average crustal composition of the North China is more felsic-rich than that of the SE China after the delamination of the lower crust of the North China with mafic components.