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摘要: 打拉二云母花岗岩岩体位于雅拉香波穹隆的东南,侵入到中生代以前的变质岩系(眼球状花岗质片麻岩和石榴黑云母片麻岩)和特提斯沉积岩(页岩和砂岩)中,主要由石英、斜长石、钾长石、黑云母和白云母组成,形成于~44.3Ma。打拉二云母花岗岩地球化学特征表明: 打拉花岗岩具有高Al2O3(16.0%~17.0%)、Na2O/K2O(>1.2)以及A/CNK比值 (>1.05),表明打拉花岗岩为富钠过铝质花岗岩; 轻稀土(LREE)富集,重稀土(HREE)相对亏损,HREE中的Ho到Lu元素有变平的特征((Ho/Lu)N= 1.11~1.46); 具有微弱或无Eu异常,Eu/Eu*=0.87~0.95; 较高的初始Sr同位素比值(87Sr/86Sr(i) =0.71754~0.71785)和较低的初始Nd比值(εNd(i)=-9.15~-12.4)。打拉花岗岩具有高Sr含量(为355×10-6~416×10-6)和Sr/Y的比值(59.1~71.5)、高La/Yb比值、低Y及HREE亏损的特征,与埃达克质花岗岩类似。上述特征表明打拉花岗岩是在较高压力条件下,以角闪岩为主的深部岩石部分熔融的结果。Abstract: The Dala two-mica granite intruded into the Yardoi gneiss dome and is located to the southeastern of the Yardoi gneiss dome. It consists of quartz, plagioclase, and K-feldspar with subordinate biotite and muscovite and formed at ~44.0Ma. Geochemical data show that the Dala granite has (1) high Al2O3 (16.0%~17.0%), Na2O/K2O ratios (>1.20) and A/CNK(>1.05); (2) highly enriched in LREE ((La/Yb)N=39.5~45.5) and depleted in HREE ((Gd/Yb)N=5.5~6.4), slightly negative or no Eu anomalies; (3) relatively high Sr concentrations (355×1010-6 ~416×10-6) and Sr/Y ratios (>59.0); (4) relatively radiogenic Sr (87Sr/86Sr(i) =0.71754~0.71785) and unradiogenic Nd (εNd(i)=-9.2~-12.4) isotope compositions, similar to those in the younger (~35Ma) leucogranites within the Yardoi gneiss dome. High Sr/Y and (La/Yb)N ratios, and Sr contents, but low Y and HREE concentrations in the Dala granite suggests it is of an adakitic composition. Our data suggest that the Dala granite was derived from partial melting of mafic lower crustal materials in the overthickened Himalayan orogenic belt, and amphibolite melting was dominant in the early stages of Himalayan magmatism.
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Key words:
- Dala pluton /
- Yardoi gneiss dome /
- Two-mica granite /
- Partial melting /
- Northern Himalayan Gneiss Dome
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