浙北地区晚中生代侵入岩锆石U-Pb年代学及Hf同位素地球化学:成因演化过程及其构造环境示踪

浙北地区位于扬子陆块东南缘,中生代岩浆侵入活动强烈,发育有何村石英二长岩、康山、沈家墈、泗岭花岗岩和凤凰山正长花岗岩等岩体。锆石LA-ICP-MS U-Pb定年结果表明,何村石英二长岩侵位时间为149.2±1.1 Ma,康山花岗岩形成于137.4±1.2 Ma,沈家墈、泗岭花岗岩和凤凰山正长花岗岩的成岩年龄在133.6±0.95 Ma~131.3±1.7 Ma之间。地球化学分析结果显示,何村石英二长岩为高钾钙碱性系列准过铝质I型花岗岩,高Rb、Th、U、K,贫Nb、Ta、Ti,中等的Eu负异常(δEu=0.78~0.79),具有类似岛弧岩浆岩的特征,锆石饱和温度为826~830℃。康山、沈家墈、泗岭花岗岩及凤凰山正长花岗岩地球化学特征相似,岩石富Si O2,高Ga、Zr、Nb和Y,贫Al2O3、Sr、Ba、Ti和P,强烈的Eu负异常(δEu=0.02~0.12);区别在于前者Fe2O3T含量为0.63%~0.89%,母岩浆温度为788~814℃,显示高分异I型特征;后三者Fe2O3T含量为1.02%~1.57%,母岩浆温度为847~868℃,属铝质A型花岗岩。锆石Hf同位素研究表明,何村石英二长岩和康山花岗岩εHf(t)变化范围分别集中在-3.79~-1.67和-6.59~-5.23之间,二阶段模式年龄(tDM2)变化范围对应于1162~1279 Ma和1350~1423 Ma,说明其源区物质可能主要来自中元古代地壳;沈家墈、泗岭花岗岩和凤凰山正长花岗岩εHf(t)主要集中在-4.24~2.48之间,tDM2主要集中于915~1161 Ma之间,说明岩体的源区物质来自于中-新元古代江南火山岛弧,εHf(t)值逐渐增大,表明越来越多的幔源物质或新生地壳参与成岩。浙北地区三类岩体分别形成于晚侏罗世古太平洋板块俯冲的挤压、早白垩世板块撤离挤压向伸展背景的转换和后造山拉张背景下岩石圈减薄等3个阶段。

Zircon U-Pb Ages and Hf Isotopic Compositions of the Late Mesozoic Intrusive Rocks in Northern Zhejiang: Implications for Genetic Evolution and Tectonic Environment

Mesozoic intrusions are widespread in northern Zhejiang, the southeastern margin of the Yangtze Block, within which five intrusions were selected for investigation, including the Hecun quartz monzonite, the Kangshan, Shenjiakan, and Siling granites, and the Fenghuangshan syenogranite. The Hecun quartz monzonite is metaluminous and high-K calc-alkalic, showing I-type features such as high LILE (Rb, K, Th, U) and relatively low HFSE (Nb, Ta, Ti) concentrations with mild negative Eu anomalies (δEu=0.78–0.79). The Kangshan, Shenjiakan, and Siling granites and the Fenghuangshan syenogranite have similar geochemical features, including relatively low Al2O3, Sr, Ba, Ti, P, extremely negative Eu anomalies (δEu=0.02–0.12), and high SiO2, Ga, Zr, Nb, Y. However, the Kangshan granite has Fe2O3T of 0.63%–0.89% and magma temperatures of 788–814 ℃, showing characteristics of highly fractionated I-type granites, whereas the Shenjiakan and Siling granites and the Fenghuangshan syenogranite have higher Fe2O3T(1.02%–1.57%) and high magma temperatures (847–868 ℃), suggestive of A-type affinity. Zircon U-Pb dating of the Hecun quartz monzonite, the Kangshan, Shenjiakan, and Siling granites and the Fenghuangshan syenogranite yielded ages of 149.2±1.1 Ma, 137.4±1.2 Ma, 133.6±0.95 Ma, and 131.3±1.7 Ma, respectively. The Hecun quartz monzonite and the Kangshan granite have εHf(t) values in ranges of -3.79 to -1.67 and -6.59 to -5.23, respectively, and crust Hf model ages (tDM2) of 1162–1279 Ma and 1350–1423 Ma correspondingly, which suggest that these granites were likely originated from remelting of Mesoproterozoic crustal materials. In contrasts, the Shenjiakan and Siling granites and the Fenghuangshan syenogranite have εHf(t) values of -4.24 to 2.48 and crust Hf model ages (tDM2) of 915–1161 Ma, which suggest that these granites were mainly originated from remelting of the Meso-Neoproterozoic Jiangnan Island Arc with substantial mantle input. Combined with previous results, we infer that the Hecun quartz monzonite was formed in a subduction setting in the late Jurassic, and the Kangshan granite formed in a compression to extension transition stage of the post-collisional environment, while the Shenjiakan and Siling granites and the Fenghuangshan syenogranite formed in an extensional setting of Early Cretaceous.