宁中白云母二长花岗岩SHRIMP锆石U-Pb年龄及岩石地球化学特征

Zircon SHRIMP U-Pb age and petrochemical and geochemical features of Mesozoic muscovite monzonitic granite at Ningzhong,Tibet.

宁中白云母二长花岗岩出露于拉萨地块中部,应用SHRIMP锆石U-Pb测年,得到锆石壳部加权平均年龄值分别为190±8Ma、193±7Ma和191±10Ma,代表成岩年龄,属早侏罗世侵入岩;核部的继承锆石年龄值大体可分为2部分:300～500Ma为古生代基底锆石;1000～1500Ma为元古宙基底锆石,个别测点得到2160Ma和2356Ma的古老年龄值,反映源岩比较复杂,并提供了拉萨地块存在中元古-古元古代结晶基底的年龄信息。岩石化学分析结果表明:白云母二长花岗岩的SiO_2为73．14%～78．12%、K_2O为3．42%～5．72%,K_2O＞Na_2 O,AL/CNK为1．22～1．42,刚玉标准分子含量为3．18%～4．69%,属典型的强过铝花岗岩。微量元素Rb含量特别高(410×10^(-6)～737×10^(-6)),Sr含量低(9．2×10^(-6)～49．0×10^(-6)),Ba含量低(20．8×10^(-6)～127×10^(-6));稀土总量低,∑REE为38．59×10^(-6)～97．84×10^(-6),铕亏损中等—强烈(δEu=0．22～0．62);同位素比值^(87)Sr/^(86)Sr为1．150900～0．861037,^(143)Nd/^(144)Nd为0．511879～0．511993,反映岩浆主要来自于上部地壳泥质岩石的部分熔融。宁中白云母二长花岗岩属于S型花岗岩,形成于同碰撞构造环境,成岩时代为早侏罗世,应属于冈底斯构造带印支造山旋回晚期碰撞阶段的产物,对它的综合研究有助于进一步认识和研究冈底斯构造带的历史和构造演化过程。

The Ningzhong granite in the central Lhasa block, Tibet, consist mainly of muscovite monzonitic granite and two-mica monzonitic granite. The zircon SHRIMP U-Pb dating for representative samples defines a crystallization age ranging from 190 ± 8Ma to 193 ± 7Ma for the muscovite monzonitic granites. The dating data indicate that the Ningzhong muscovite granite is the oldest, known in the Tibet plateau. These rocks have ratively high contents of SiO2 （73.14% - 78.12% ） and K2O （3.42% - 5.72% ）, with ratio of CaO/Na2O （0.12 -0.28） and AL/CNK （1.22 -1.42）, showing strongly peraluminous feature. There is an evident negative Eu anomaly （δEu = 0.22 - 0.62） in its REE. In the trace element, Rb = 410 × 10 ^-6 - 737 × 10^ -6 and Sr = 9.2 ×10^ -6 - 49.0 × 10 ^-6, Ba=20.8 × 10^-6 - 127 × 10^- 6. In the Sr and Nd isotopes, ^87Sr/^86 Sr = 1. 150900 -0. 861037 and ^143Nd/^144 Nd =0. 511879 - 0. 511993. The above features show that the magma of the Ningzhong muscovite monzonitic granite was derived mainly from the melting of an upper crust source rocks, dominated by argillaceous rocks. The Ningzhong muscovite monzonitic granite is S-type granite and have a presumable relationship with the Indosinian collisional orogeny in the Gangdise.