Zircon U-Pb dating, Hf isotope characteristics of Late Jurassic granites in eastern Jiaodong Peninsula and their constraints on metallogenetic tectonic setting of gold deposit

Abstract

胶东地区是目前我国最重要的金矿产地，已累积探明黄金储量接近5000t。金矿在胶东地区分布非常不均，主要集中在胶北地体，而胶东东部地区金矿产出较少。晚侏罗世的玲珑花岗岩是胶北金矿区最重要的赋矿围岩之一。虽然胶东东部同样有晚侏罗世花岗岩发育，但赋存金矿较少，因此前人对东部这些花岗岩体的研究相对较少。本文选取胶东东部的文登岩体和垛崮山岩体进行了锆石U-Pb定年和Hf同位素分析，同时收集整理了胶北地体的玲珑岩体数据，通过开展胶东东部及西北部同一时期岩体的差异性研究，为该区金成矿作用的研究提供重要线索。分析结果显示，文登岩体和垛崮山岩体的形成时代为晚侏罗世（~160Ma），与玲珑岩体时代一致。三个岩体均含有较多的三叠纪（~230Ma）和新元古代（~780Ma）的继承锆石，表明晚侏罗世花岗岩的岩浆源区主要为经历了超高压变质作用的扬子板块。不同的是，玲珑岩体中有更多的华北板块物源的年龄记录（~1400Ma、~1700Ma、~2500Ma），垛崮山岩体中这些年龄的继承锆石少于玲珑岩体，而文登岩体更少。文登岩体、垛崮山岩体的新生岩浆锆石ε_Hf（t）值范围分别为-30.7~-20.3和-27.5~-17.9，玲珑岩体的新生岩浆锆石ε_Hf（t）值的变化范围更广，在-29.3~-9.3之间，而且呈现出从文登岩体到垛崮山岩体到玲珑岩体，ε_Hf（160Ma）值逐渐增高的现象。其原因可能是扬子板块与华北板块碰撞后，幔源岩浆底侵并诱发加厚的地壳发生部分熔融，形成花岗岩。在此过程中胶北地体深部受到幔源物质的影响，从而对金及成矿相关元素产生一定的预富集作用，而同时期胶东东部地区未受到明显的地幔物质影响，可能是该区金矿产出较少的原因之一。;Jiaodong Peninsula is the most important gold producing province in China, with an explored reserve of nearly 5000t gold. However, the gold deposits are very unevenly distributed in this area, e.g., most in Jiaobei terrane but less in eastern Jiaodong Peninsula. Late Jurassic Linglong pluton is one of the important ore-hosting rocks in Jiaobei terrane. Although Late Jurassic granites also exist in eastern Jiaodong Peninsula, gold deposits are much less abundant in them. Thus, relatively few studies were focused on these granite plutons in the eastern Jiaodong Peninsula. Here, we carried out zircon U-Pb dating and Hf isotopic analysis on the Wendeng and Duogushan plutons in eastern Jiaodong Peninsula to compare them with those of the Linglong pluton in Jiaobei terrane to seek the differences of the gold mineralization in the Late Jurassic granites between these two areas. Our results show that the Wendeng and Duogushan plutons were formed at Late Jurassic (~160Ma), which is consistent with those of the Linglong pluton. The inherited zircons from the three plutons are all dominated by Triassic (~230Ma) and Neoproterozoic (~780Ma) ages, indicating their magmatic source mainly derived from crustal materials of Yangtze Craton, which had undergone ultrahigh-pressure metamorphism in Triassic. The difference among them is the number of inherited zircons relating to North China Craton (~1400Ma,~1700Ma,~2500Ma), which are most abundant in Linglong pluton, less in Duogushan pluton, and least in Wendeng pluton. Magmatic zircons from the two plutons in eastern Jiaodong Peninsula have variably negative ε_Hf(160Ma) values, from -30.7 to -20.3 for Wendeng pluton, and from -27.5 to -17.9 for Duotgushan pluton. Contrastively, the range of ε_Hf(160Ma) values of Linglong pluton is significantly wider, i.e., from -29.3 to -9.3. Moreover, the ε_Hf(160Ma) values also increase gradually from the Wendeng pluton to the Duogushan pluton, and then to the Linglong pluton. The possible interpretation of this trend is that after the collision of the Yangtze Craton and North China Craton in Triassic, the underplating of mantle material induced partial melting of the thickened crust as early as Late Jurassic. Meanwhile, the Jiaobei terrane was influenced by mantle materials, which promoted the pre-enrichment of gold. In contrast, the eastern Jiaodong Peninsula was lack of involvement of mantle material in Late Jurassic, which might be one of the reasons for fewer gold deposits in the area.