西藏冈底斯南缘班则石英二长闪长岩锆石U-Pb年龄、Lu-Hf同位素组成及其地质意义

Zircon U-Pb Age,Composition of the Lu-Hf Isotopic and Its Geological Significance of Quartz Monzodiorite in Banze,the South Margin of Gangdise in Tibet

班则石英二长闪长岩体出露于西藏冈底斯花岗岩带南缘班则南西一带。该岩体SiO_(2)含量介于60.64%~65.36%,属中酸性岩类,Al_(2)O_(3)含量平均为16.22%,ALK平均为6.53%,里特曼指数σ介于2.04~2.29,岩石属准铝质高钾钙碱性系列。轻稀土富集,δEu略显负异常,稀土配分曲线为LREE富集的右倾型。在原始地幔标准化的蜘蛛网图中,相对富集大离子亲石元素Rb、Th、U、K,亏损高场强元素Ta、Nb、P、Ti。锆石LA-ICP-MS U-Pb测年表明其形成年龄为(48.75±0.64)Ma,时代为始新世。锆石ε_(Hf)(t)均为正值,两阶段Hf模式年龄(t_(DM2))介于272.6~483.8 Ma。地质背景、地球化学及同位素特征显示,班则始新世石英二长闪长岩体形成于同碰撞后期,其形成过程中源区存在亏损幔源物质,古生代中期可能是冈底斯中段南缘地区重要的地壳增生期。

The quartz monzonite diorite is exposed in the southern margin of the Gangdise granite belt in Tibet,the southwest area of Banze.The SiO_(2)content of this rock ranges from 60.64%to 65.36%,it’s a medium-acid rock.The average Al_(2)O_(3)content is 16.22%,the average ALK is 6.53%,and theσis between 2.04 and 2.29.The rock belongs to meta-aluminum and high potassium Calcium alkaline series.The light rare earth is enriched,theδEu is slightly negative anomaly,and the rare earth partition curve is a right-sloping type of LREE enrichment.In the normalized spider web map of the primordial mantle,relatively enriched with large-ion-lithophile elements Rb,Th,U,K,depleted of high-field-strength elements Ta,Nb,P,Ti.Zircon LAICP-MS U-Pb dating shows that its age is(48.75±0.64)Ma,the era is the Eocene.ZirconεHf(t)are all positive values,the two-stage Hf model ages(tDM2)ranged from 272.6 Ma to 483.8 Ma.The geological background,geochemistry and isotopic characteristics show that the Banze Eocene quartz monzonitic diorite was formed in the late syn-collision period,and there were depleted mantle-derived materials in the source area during the formation process,the middle Paleozoic may be an important period of crustal accretion in the southern margin of the middle Gangdise.