苏鲁造山带胶东区段花岗片麻岩类的常量与微量元素地球化学:扬子克拉通北缘新元古代活动大陆边缘的证据  

Major and trace element geochemistry of granitic gneisses from Sulu orogen,eastern Shandong Peninsula: evidence for a Neoproterozoic active continental margin in the northern margin of the Yangtze craton.

作　　者：薛怀民[1] 刘福来[1] 孟繁聪[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2006年第7期1779-1790,共12页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目"973"(编号2003CB716502);国家自然科学基金重大项目(编号40399143);中国地质调查局地质大调查项目(121201056606)资助成果.

摘　　要：胶东东部地区的基底片麻岩以牟平-海阳断裂为界,两侧在岩性组合和地球化学性质上完全不同,本文认为该断裂应代表苏鲁造山带的西北边界(北段)。断裂西侧岩性包括花岗闪长质、奥长花岗质和花岗质,地球化学上表现为低硅(SiO2 =64．16-71．76％)、高铝(Al2O3=15．60-18．51％)、显著的Ba和Sr正异常、强烈的稀土元素分馏程度(LaN／YbN=15．77- 68．19)和贫重稀土元素(YbN=2．9-4．4)、Eu异常不明显(8Eu=0．86-1．02),具典型的太古代高铝TTG岩石组合的特征。而东侧新元古代的基底片麻岩从地球化学上可分为高钾的Ⅰ型花岗质片麻岩和富钠的A型花岗质片麻岩。前者总体表现为富钾(K2O／Na2O比接近或大于1)和亚铝质,强烈亏损Sr、Nb和Ta,轻稀土元素之间的分馏程度强(LaN／SmN=4．21～5．37),而重稀土元素之间几乎无分馏(CdN／YbN=0．78-1．54),负Eu异常较强(δEu=0．47-0．61),岩石地球化学特征显示它们的原岩类似于活动大陆边缘的Ⅰ型花岗岩类。而后者则以偏碱和富钠(Na2O／K2O=1．06-1．77),高Y(20．4—52．9μg/g)、Zr(218—390μg/g)、Ga(18．8—22．9μg/g)及稀土元素丰度为特征,稀土元素分馏程度强(LaN／YbN=11．30-19．09),弱到中等程度的负Eu异常(δEu=0．94-0．65),强烈亏损Sr为显著特征,Nb和Ta相对于La也强烈亏损,而Ba则表现出明显的正异常。推测Ⅰ型花岗岩是在与俯冲有关的构造环境下,压力0．8-1．0Gpa条件下,由受俯冲板片脱水交代的镁铁质下地壳(26- 33km)部分熔融形成的。而A型花岗岩则是在Ⅰ型花岗质岩浆形成后,由脱水的紫苏辉石质残留下地壳在温度大于900℃的条件下再一次部分熔融形成的。相对于Ⅰ型花岗岩,A型花岗岩中可能有更多的大洋岩石圈的组份被卷入。The petrological assemblage and geochemical characteristics of the basement gneisses from two sides of the MoupingHaiyang fault are distinctly different, we guess here that the fault is probably the northwestern margin of the Sulu orogen （ northern segment）. The petrological assemblages of the basement gneisses from the western side of the Mouping-Haiyang fault include granodioritic, trondhjemitic and granitic, are characterized by low SiO2 （64. 16 - 71.76%） and high Al2O3 contents （15.60 - 18. 51% ） , strong positive Ba and Sr anomalies, strong fractionation between LREE and HREE （ LaN/YbN = 15.77 -68. 19） , strong depletion in HREE （ YbN = 2.9 - 4. 4 ） and the absence of significant Eu anomaly （ ^δEu = 0. 86 - 1.02 ） , which are similar to the typical Archaean TTG gneisses. Whereas the Neoproterozoic basement gneisses eastern to the Mouping-Haiyang fault define two distinct types in chemical compositions, high-K Ⅰ-type granitic gneisses and Na rich A-type granitoid gneisses. The Ⅰ-type granitic gneisses are generally K rich （ K2O/Na2O ratio near or greater than 1 ） and metaluminous, and characterized by strong depletion in Sr, Nb and Ta, with relatively strong negative Eu anomaly（ δEu = 0. 47 - 0. 61 ）, the fractionations among LREE are strong （ LaN/SmN = 4. 21 - 5.37）, whereas almost no fractionation among HREE （GdN/YbN = 0. 78 -1.54）. The geochemical characteristics of the Ⅰ-type granitic gneisses are similar to those of granitoid from modern active continental margins. Whereas the A-type granitic gneisses are characterized by high alkali and Na rich （ Na2O/K2O = 1.06 - 1.77）, high Y （20.4 -52.91μg/g）, Zr （218 - 390μg/g）, Ga （ 18.8 -22.9μg/g） and REE contents, strong fractionation between LREE and HREE （LaN/YbN = 11.30 - 19.09）, with slightly to medium negative Eu anomaly （ δEu = 0.94 - 0.65 ） , strong depletion in Sr, Nb and Ta, whereas significant positive Ba anomaly. Thus, the Ⅰ-type granite is most probably formed by partial mel

关 键 词：苏鲁造山带 花岗质片麻岩 Ⅰ型花岗岩 A型花岗岩 新元古代 大陆边缘 

分 类 号：P588.121[天文地球—岩石学] P588.345[天文地球—地质学]