Petrogenesis and tectonic setting of the late Early Cretaceous Kong Co A-type granite in the northern margin of Central Lhasa Subterrane, Tibet

Abstract

在中拉萨地块北部尼玛控错地区发育着一套碱性长石花岗岩，对该花岗岩体开展成因和形成背景的研究，能为探索班公湖-怒江洋的构造演化提供有价值的信息。用LA-ICP-MS方法测得该花岗岩的锆石²⁰⁶Pb/²³⁸U年龄加权平均值为104.9±1.4Ma （MSWD=1.5）和104.6±1.3Ma （MSWD=1.3），表明该岩体形成于早白垩世。花岗岩具有高硅（SiO₂=76.75%~77.51%，平均77.27%）、高钾（K₂O=4.61%~4.85%，平均4.77%）、高碱（K₂O+Na₂O=8.24%~8.57%，平均8.44%）、低钙（CaO=0.28%~0.48%，平均0.35%）、低镁（MgO=0.11%~0.16%，平均0.13%）和低铝（Al₂O₃=11.79%~12.22%，平均12.09%）等特征，里特曼指数（σ）为1.96~2.15（平均2.08），A/NK值为1.06~1.09，A/CNK值为1.01~1.04。这些特征表明控错花岗岩为弱过铝质的高钾钙碱性-钾玄岩系列岩石。控错花岗岩相对富集Zr、Nb、Ce、Y和Hf等微量元素，相对亏损Ti、Ba、Sr和P等微量元素，分异系数（DI）为95.5~96.9（平均：96.3），还具有较高的FeO^T/MgO值（5.61~10.22，平均7.26）、10000Ga/Al值（2.78~2.56，平均2.84）、Y/Nb值（2.29~4.97，平均3.57）、Rb/Nb值（11.6~18.2，平均15.2）；此外，该岩体还具有较高的全岩Zr饱和温度（875~910℃，平均890℃）和锆石Ti饱和温度（848~919℃，平均890℃），明显的Eu负异常（δEu=0.04~0.09，平均0.06），以及向右缓倾的Ⅴ型稀土元素配分曲线，这些特征表明控错花岗岩为产于碰撞后环境的A2型花岗岩。正的锆石ε_Hf（t）值（4.26~6.38，平均5.16）、相对年轻的锆石Hf地壳模式年龄（t_DM2=757~889Ma，平均833Ma）、下地壳与地幔混合特征的（⁸⁷Sr/⁸⁶Sr）_t（0.7194~0.7407，平均0.7313）、ε_Nd（t）（-3.39~-3.00，平均-3.24））和Pb同位素特征（（²⁰⁶Pb/²⁰⁴Pb）_t=18.792~18.845，（²⁰⁷Pb/²⁰⁴Pb）_t=15.708~15.718，（²⁰⁸Pb/²⁰⁴Pb）_t=38.870~38.037），指示控错花岗岩熔融于幔源物质加入的新生地壳。研究结果揭示，控错花岗岩形成于羌塘-拉萨地块碰撞作用下，俯冲板片的断离后，软流圈上涌诱发的地壳部分熔融，并经历了显著的以钾长石和角闪石为主的分离结晶作用。;A set of alkali feldspar granite develops well in the Kong Co of Nyima County in the northern margin of Central Lhasa Subterrane, Tibet, which can provide invaluable information to explore the geological evolution of the Bangong Co-Nujiang Ocean. The weighted averaged zircon ²⁰⁶Pb/²³⁸U ages of the granite measured by LA-ICP-MS are 104.9±1.4Ma (MSWD=1.5) and 104.6±1.3Ma (MSWD=1.3), respectively, indicating that it was formed in the late phase of Early Cretaceous magmatism. The rocks in Kong Co granite are grayish-white, with fine-grained equigranular structure and medium coarse-grained porphyritic-like texture. The rock-forming minerals mainly include quartz, potassium feldspar, plagioclase, and a small amount of transparent accessory minerals such as biotite and amphibole, as well as a few melanocratic mineral like magnetite. The granite is characterized by high contents of SiO₂ (6.75%~77.51%), K₂O (4.61%~4.85%), and K₂O+Na₂O (8.24%~8.57%), with low contents of CaO (0.28%~0.48%), MgO (0.11%~0.16%) and Al₂O₃ (11.79%~12.22%), and a Rittman index (σ₄₃) of 1.96~2.15 and A/CNK ratios of 1.01~1.04. These characteristics reveal that the Kong Co granite is weakly peraluminous high potassium calc-alkaline-potassium basalt series rock. The granite is relatively enriched in Zr, Nb, Ce, Y and Hf, and deficient in Ti, Ba, Sr and P. It shows a highly variation of index (DI:95.50~96.90), and also has high values of FeO^T/MgO (5.61~10.22), 10000Ga/Al (2.78~2.56), Y/Nb (2.29~4.97), Y/Nb (2.29~4.97), Rb/Nb (11.6~18.2), which is consistent with A2-type granites produced in post collision environment. In addition, the rock has high whole-rock Zr saturation temperatures (875~910℃) and zircon Ti saturation temperatures (848~919℃), with a significant negative Eu anomaly (δEu=0.04~0.09), and a Ⅴ-shaped rare earth element curve gently inclined to the right. These characteristics also prove the Kong Co granite is A2 type granite. They have relatively positive ε_Hf(t) values (4.26~6.38), and two-stage Hf model ages (t_DM2) ranging from 757Ma to 889Ma, with (⁸⁷Sr/⁸⁶Sr)_t=0.7194~0.7407 and ε_Nd(t)=-3.39~-3.00, and Pb isotopic ((²⁰⁶Pb/²⁰⁴Pb)_t=18.792~18.845, (²⁰⁷Pb/²⁰⁴Pb)_t=15.708~15.718, (²⁰⁸Pb/²⁰⁴Pb)_t=38.870~38.037) characteristics of lower crust-mantle mixture. These data indicate that the granite was sourced from interaction between mantle-derived and juvenile lower crust-derived melts. This occurred when the post collision extension of Qiangtang and Lhasa terranes the subduction plate split to create a slab window, and rising asthenosphere triggered re-melting of the lower crust basalt, resulting in the formation of the late Early Cretaceous A-type granite around Kong Co.