新元古代扬子北缘地壳增生事件：来自汉南祖师店奥长花岗岩地球化学、锆石LA-ICP-MS U-Pb年代学和Hf同位素证据

Neoproterozoic Crustal Accretion of the Northern Margin of Yangtze Plate:Constrains from Geochemical Characteristics,LA-ICP-MS Zircon U-Pb Chronology and Hf Isotopic Compositions of Trondhjemite from Zushidian Area,Hannan Region

扬子地块北缘大面积出露的超基性—基性—酸性侵入岩被称为"汉南杂岩"。其中的酸性侵入岩主要由二里坝、五堵门、祖师店等岩体构成。本文研究的祖师店岩体为一套奥长花岗岩,其地球化学特征表现为高Si(SiO 2=73.18%~77.18%)、富Na(Na2O=4.55%~5.49%)、低K(K2O=2.01%~3.01%)、Mg(MgO=0.14%~0.25%),属于弱过铝质中钾钙碱性花岗岩。稀土元素球粒陨石标准化图解呈右倾平坦型,Eu略具正异常。样品标准化图解显示富集Rb、Ba、Th等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素;(Sr/Y)为28.16~50.05;(La/Yb)N为7.04~14.21。地球化学特征显示该岩体具岛弧花岗岩特征。与二里坝、五堵门岩体对比后发现祖师店岩体与Adakite不一致。高Sr低Y的特征指示其形成压力相对较大,残留相为石榴子石和金红石、无斜长石。运用锆石饱和温度计和锆石钛温度计计算出这套花岗岩结晶温度为680~694℃,属于玄武质岩石低程度部分熔融的产物。LAICP-MS锆石U-Pb定年结果获得728±3 Ma的成岩年龄和786±4 Ma的捕获锆石年龄,捕获锆石边部记录了728 Ma的岩浆活动事件。锆石Hf同位素分析结果显示,n(176Hf)/n(177Hf)初始值为0.282470~0.282683,εHf(t)=+5.0^+12.2,接近同时期亏损地幔演化线的εHf(t)。两阶段模式年龄(TDM2)为870~1324 Ma,主体集中于约1100 Ma,代表新元古代扬子北缘一期地壳增生事件。推测该岩体是由新生玄武质洋壳俯冲消减过程中在含水条件下部分熔融形成的。结合该区已有大量花岗岩研究成果,提出汉南杂岩中花岗质岩石的时空关系反映岛弧演化和迁移的历史过程,祖师店岩体代表洋内非成熟岛弧向成熟岛弧转化阶段的产物。

A large area of ultrabasic—basic—acid rocks which are Called ＂ Hanna Complex＂ outcropped in the northern margin of Yangtze Plate.The acid plutons mainly exposed in Erliba,Wudumen,and Zushidian areas.The Zushidian pluton is a suit of trondhjemite characterized by high contents of Si （SiO2 =73.18% ～ 77.18%） and Na （Na2O =4.55 % ～ 5.49%）,low contents of K （K2O =2.01% ～ 3.01%） and Mg （MgO =0.14% ～ 0.25%）,belonging to slight peralurninous and medium-K calc-alkaline series.The chondrite-normalized REE patterns are charaeterized by smoothly REE right-deviation and slightly positive Eu anomaly.The primitive mantlenormalized spidergrams are also charaeterized by enrichment of LILE （Rb,Ba and Th） and depletion of HFSE （Ni,Ta and Ti）,with Sr/Y and （La/Yb） ratios of 28.16 ～ 50.05 and 7.04 ～ 14.21,respectively.Geochemical data shows that the pluton has the feature of arc granite.The Zushidian pluton is not adakite compared to the Erliba and Wudumen plutons.The characteristics of high Sr and low Y indicate that the formation pressure for the Zushidian pluton is relatively high,with residues of garnet and rutile,and without plagioclase.By Using saturated zircon thermometer and zircon titanium thermometer,the crystallization temperature for the Zushidian pluton can be constrained at 680 ～ 694℃,resulting from low partial melting of basaltic rocks.LA-ICP-MS zircon U-Pb dating yield the formation age of 728 ± 3 Ma for the Zushidian pluton and the captured zircon age of 786 ± 4 Ma,and the edge of captured zircons also recorded the ca.728 Ma magmatic event.The initial,n（176Hf）/n（177Hf） values of zircons range from 0.282470 ～0.282683,corresponding to εHf（t） =＋5.0 ～ ＋ 12.2.Zircon two-stage Hf model ages （TDM2） range from 871 Ma to 1325 Ma,with a peak age of ca.1100 Ma,which represents a crustal accretion event in the northern margin of Yangtze Plate at Neoproterozoic.In this paper,we speculate that the rocks possibly formed in the oceanic crust subduction setting,and resulted from the partial melting of juvenile oceanic crust with water added.Combining with the available data,we propose that the time—space relationship of granitic rocks in the Hannan area reflect the evolution and migration processes of island arcs in the north margin of Yangtze Plate during Neoproterozoic,and the Zushidian trondhjemite was formed at the island arc transformational stage that the immature arc to the mature marginal island arc.