扬子地台北缘檬子地区侵入岩年代格架和岩石成因研究

Zircon U-Pb age and petrogenesis of intrusions from Mengzi area in the northern margin of Yangtze plate

对扬子地台北缘檬子地区发育的辉长苏长岩、角闪辉长岩、花岗闪长岩、石英闪长岩和钾长花岗岩体进行了LA-ICPMS锆石U-Pb定年和Lu-Hf同位素、全岩主量和微量元素分析。结果表明,该区基性和中酸性岩体均形成于新元古代中晚期。其中基性侵入体辉长苏长岩形成于764±38Ma,角闪辉长岩形成于757±32Ma,均属钙碱性系列,轻稀土富集,微量元素原始地幔蛛网图中显示富集大离子亲石元素(LILE,如Ba、La、Ce、Sr等)、具有明显的Nb、Ta负异常和弱的P、Ti具负异常,形成于地幔原始石榴石二辉橄榄岩源区,部分熔融的程度大于10%。锆石Lu-Hf同位素研究揭示辉长苏长岩ε_(Hf)(t)值变化于+6.92～+13.99,tDM变化于736～1058Ma,当ε_(Hf)(t)为最大正值时,对应的tDM与岩石的形成年龄基本一致;角闪辉长岩εHf(t)变化于+4.74～+14.97,tDM主体范围变化于707～1214Ma,当ε_(Hf)(t)具有最大的正值时,tDM接近于岩石形成年龄。反映出辉长苏长岩和角闪辉长岩在形成过程中,经历了地壳混染作用,最有可能是中元古代地壳的混染。对石英闪长岩年代学测试表明其形成于774±34Ma。花岗闪长岩、石英闪长岩属钙碱性系列,具有相似的稀土和微量元素分布特点,表现为大离子亲石元素富集,亏损高场强元素(Ta、Nb、Y、Yb等)的特点。钾长花岗岩样品属高钾钙碱系列,岩石稀土元素丰度较低,∑REE=105.9×10^(-6)～133.1×10^(-6),轻稀土富集,具有强烈的Eu负异常(δEu=0.03～0.09),微量元素原始地幔标准化表现为大离子亲石元素富集,Ba、Nb、Sr、P、Ti具有明显负异常。石英闪长岩ε_(Hf)(t)变化于-4.46～+13.10之间,大部分为正值,平均值为+4.80,t2DM变化于820～1726Ma,平均值为1256Ma,显示其主要为新元古代新生地壳部分熔融的产物,在形成过程中有中元古代地壳物质的参与;钾长花岗岩ε_(Hf)(t)主体为负值,变化于-16.49～+8.33,平均值为-5.26,t_(2DM)为943～2080Ma,平均值为1458Ma,显示其主体为古老地壳物质部分熔融的产物,新元古代新生地壳参与较少。结合区域地质特征分析,檬子地区基性和中酸性侵入体形成于活动大陆边缘弧环境,是新元古代勉略洋盆向南俯冲的地质记录。

The combined U-Pb and Lu-Hf in situ LA-ICPMS analyses of zircons and the geochemistry studies reveal that the gabbronorite, bojite, granodiorite, quartz diorite and moyite along the northern margin of the Yangtze plate are all formed during the Middle-Neoproterozoic and Late-Neoproterozoic. The gabbronorite is formed at 764±38Ma and the bojite is formed at 764±38Ma. Both of them are calc-alkaline series and rich in LREE. The spider diagrams of the trace elements show that they are rich in large ion lithophile element with a distinct Nb-Ta negative anomalies and smaller negative P, Ti anomalies. The geochemistry features suggest that both of the basic intrusions are formed by partial melting （more than 10%） of the garnet iherzolite. The zircon Hf-isotopic analyses of the gabbronorite suggest that the εHf（t）values range between ＋6.92 and ＋13.99, the tDM values are mainly between 736Ma to 1058Ma. The tDM value is generally similar to the concordant U-Pb age when the εHf（t）value is the highest. The εHf（t）values of the zircons from bojite range between ＋4.74 and ＋14.97, the tDM values are mainly between 707Ma to 1214Ma and the tDM value is also similar to the concordant U-Pb age when the εHf（t） value is the highest. This reflects that the gabbronorite and bojite have experienced significant crustal contamination, most likely Meso-proterozoic crustal contamination. Besides, the quartz diorite is formed at 774±34Ma. The granodiorite and quartz diorite are calc-alkaline series, both of them have similar geochemical features and are rich in LILE and depleted in HFSE. The moyite belong to high-K calc-alkaline series. Its abundance of the REE is smaller （∑REE =105.9×10-6~133.1×10-6） and it is rich in LREE with a strong negative Eu anomaly. The primitive mantle-normalized trace element diagrams show enrichments in LILE and strong negative anomalies for Nb, Sr, P and Ti. The zircon Hf-isotopic analyses of quartz diorite suggest that the εHf（t） values range between -4.46 and ＋13.10, most of which are positive value （the average value is ＋4.80）,the t2DM values are mainly between 820Ma to 1726Ma. All of that reveal the intrusion is the produce of the partial melting of Neoproterozoic juvenile crust, with Mesoproterozoic crustal material adding up to it. The εHf（t） values of zircons from the moyite range between -16.49 and ＋8.33, which generally are negative （the average value is -5.26）, the t2DM values are mainly between 943Ma to 2080Ma （the average value is 1458Ma）. This shows that the moyite is the produce of the partial melting of ancient crust with little additions of Neoproterozoic juvenile crust. Consistent with the characteristics of regional geology, the intrusions are formed in an active continental margin tectonic setting, which is the geological record of south-directed subduction by Mianlue oceanic crust in Neoproterozoic.