摘要
沙河湾、曹坪和柞水花岗岩体位于商丹与勉略缝合带之间的南秦岭中部地区,三个岩体Al_2O_3含量在14.08%～15.85%之间,相对富硅、富镁、富碱,K_2O与Na_2O含量相近,属于钾玄-高钾钙碱性系列准铝质Ⅰ型花岗岩。它们均富集Rb、Sr、Ba等大离子亲石元素和轻稀土,亏损Nb、Ta等高场强元素和重稀土以及Y,铕负异常不明显(δEu=0.62～0.91),显示同源岩浆演化特征。沙河湾、曹坪、柞水岩体的锆石LA-MC-ICPMS铀铅年龄分别为212±0.93 Ma、224.1±1.1 Ma和224.8±1.1 Ma;锆石ε_(Hf)(t)值的变化范围分别是-1.7～+5.0(212Ma)、-6.0～+3.3(224Ma)和-5.8～+4.4(224Ma),前者高于后两者;三者的Hf同位素二阶段模式年龄(t_(DM2))在791～1366Ma之间,平均值为1075Ma。这些特征表明这些花岗岩是源于亏损地幔的岩浆与源于中.新元古代或更老下地壳的岩浆发生混合作用的产物。沙河湾、曹坪和柞水岩体形成在华北与华南板块碰撞后,是秦岭造山带从挤压向伸展转变的地球动力学背景下岩石圈拆沉的结果,而秦岭造山带印支晚期岩石圈构造体制应以挤压向伸展转变为主,岩石圈构造属性从挤压向伸展转变事件可能发生在224～210Ma之间。
Shahewan, Caoping and Zhashui granitic plutons are located in the center of the South Qinling orogenic belt, Central China. These three plutons have Al(2)O(3) contents ranging 14.08% similar to 15.85%, and relatively high contents of SiO(2),MgO and alkali, with similar contents of K(2)O and Na(2)O, showing petrochemical features of metaluminous I-type granites of shoshonitic or high-K calc-alkaline series. This is supported by enrichment in LREE and LILE (e. g. Rb, Sr, Ba), slightly negative Eu anomaly (delta Eu = 0.62 similar to 0.91), and depletion in HREE, Y and HFSE (e. g. Nb, Ta), which suggest a similar magma source. Analysis using LA-MC-ICPMS yields zircon U-Pb ages of 212 +/- 0.93 Ma, 224.1 +/- 1.1 Ma and 224.8 +/- 1.1 Ma for Shahewan, Caoping and Zhashui plutons respectively, with The epsilon(Hf) (t) values are -1.7 similar to +5.0 for Shahewan (212 Ma), -1.7 similar to +5.0 for Caoping (224 Ma) and -5.8 similar to +4.4 for Zhashui (224 Ma), with the former slightly higher than two latter. The two-stage Hf-isotope model ages (t(DM2)) of all three plutons range from 791 to 1366 Ma, and average 1075 Ma. This suggests that they were products of two end-member mixing magmas respectively sourced from depleted mantle and from lower crust of Neoproterozoic or earlier ages. Combined with regional investigation, we argue that these Late Triassic granites were formed by lithospheric delamination during tectonic transition from compression to extension, which followed the continental collision between North China and South China plates. Thus, the Late Triassic tectonic nature of lithosphere in Qinling orogenic belt must be characterized by transition from compression to extension; and the tectonic transition possibly occurred in the period of 224 similar to 210 Ma.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第2期248-264,共17页
Acta Petrologica Sinica
基金
国家重大基础研究项目(2006CB403502)
国家自然科学基金项目(40872071)
西北大学大陆动力学国家重点实验室科技部项目(BJ081344)联合资助
作者简介
弓虎军,男,1971年生,博士,岩石地球化学,E-mail:hujungong@163.com 朱赖民,通讯作者:教授,博士,主要从事矿床地球化学研究和教学,E-mail:zhulaimin@nwu.edu.cn
