摘要
北山南部晚古生代构造背景争议已久,该区下二叠统普遍发育一套流纹岩系,但尚未对其开展系统的年代学与地球化学研究。本文首次对该区下二叠统5条代表性剖面中的流纹岩进行了元素地球化学、锆石U-Pb定年、全岩Sr-Nd及锆石Hf同位素等研究。U-Pb年龄介于294. 7～272. 7Ma之间,结合古生物资料,明确其喷发时代为早二叠世。该区早二叠世流纹岩大多属高钾钙碱性系列,呈钙碱-碱钙质,为准铝-过铝质岩石。岩石主量元素Si、K、Na、Fe含量高,贫Ca、Mg;微量元素富集Ga、Rb、Zr、Hf、Y,亏损Ba、Sr等;稀土元素存在明显负Eu异常;呈现高Fe OT/MgO与10000×Ga/Al比值,这些特征表明该区早二叠世流纹岩为A型流纹岩。独山剖面流纹岩锆石具有高的εHf(t)值(+11. 4～+16. 7),Hf模式年龄(tDM2)为231～573Ma;干泉剖面流纹岩锆石εHf(t)值为-8. 8～+8. 9,Hf模式年龄(tDM2)为802Ma～1961Ma;珊瑚井和俞井子剖面中的流纹岩具有低的εNd(t)值(-4. 21～-2. 92),高的(87Sr/86Sr)i值(0. 7108～0. 7223),Nd亏损地幔模式年龄(tDM)为1256～1386Ma,其锆石εHf(t)为正值(+1. 3～+17. 0)(tDM2为691～1797Ma);沙红山南剖面流纹岩εNd(t)值(-2. 00～-1. 81)较低,Nd亏损地幔模式年龄(tDM)为1207～1222Ma,锆石εHf(t)值介于+1. 6～+10. 7(tDM2为623～2576Ma)。地球化学特征表明独山剖面的流纹质岩浆为古生代新生地壳的重熔,其它剖面酸性火山岩岩浆则主要为中新元古代地壳物质熔融所形成,揭示了北山南部早二叠世岩石圈伸展减薄的地球动力学背景,结合该区构造岩浆事件序列、地层学以及沉积学等其它地质学证据,认为北山南部早二叠世为裂谷环境。
There are still 0n going debates with respect to Late Paleozoic tectonic issues in southern Beishan orogen,NW of China.For instance,the question as to whether the accretion of the Paleo-Asian Ocean persisted through the Permian or completed before this period remains enigmatic. In order to better understand the tectonic evolution in this area,a combined study of geochemistry,whole rock Sr-Nd and zircon U-Pb-Hf isotopes has been carried out for the rhyolites collected from five representative Lower Permian sections in this area. LA-ICP-MS zircon U-Pb dating results show the formation age of these rhyolites are 294. 7 - 272. 7 Ma（206 Pb/238 U ages）.Combined with the stratigraphy and paleontological data,it can be confirmed that the rhyolite suite formed during the Early Permian.Most of rhyolite samples in the research area,which are regarded as the high-K calc-alkaline series with the characteristics of alkalicalcic and calc-alkalic,are metaluminous to peraluminous. They exhibit similar geochemical characteristics including enrichment in Si,K,Na,Fe,poor Ca and Mg,positive Ga,Rb,Zr,Hf,Y and negative Ba,Sr,Eu anomalies,with high Fe OT/MgO and 10000 × Ga/Al values. These geochemical features indicate that the rhyolites are categorized as A-type granites. The rhyolites from Dushan section display extremely high zircon εHf（ t） values varying from ＋ 11. 4 to ＋ 16. 7 with the Hf model ages（ tDM2） ranging from 231 Ma to573 Ma. In contrast,the zircon εHf（ t） values vary from-8. 8 to ＋ 8. 9 for the rhyolites from Ganquan section,and the Hf model ages（ tDM2） vary from 802 Ma to 1961 Ma. The rhyolites from Yujingzi and Shanhujing sections display low εNd（ t）（-4. 24 --2. 92）,high（87 Sr/86 Sr）i（ 0. 7108 - 0. 7223） and positive zircon εHf（ t） values（ ＋ 1. 3 - ＋ 17）,with Nd model ages（ tDM） of 1256 -1386 Ma and Hf model ages（ tDM2） of 691 - 1797 Ma. The rhyolites from the section in southern Shahongshan have low εNd（ t）（-2. 00--1. 81） and positive zircon εHf（ t） values（ ＋ 1. 6 - ＋ 10. 7）,with Nd model ages（ tDM） of 1207 - 1222 Ma and Hf model ages（ tDM2） of 623 - 2576 Ma. Therefore,it can be concluded that The rhyolites from Dushan section were derived from partial melting of Paleozoic crust,but the felsic volcanics from the other sections evolved from partial melting of Middle-Late Proterozoic continental crust. The A-type rhyolites resulted from crustal stretching and thinning activities during the Early Permian in southern Beishan.Combining with other geological evidence（ e. g.,tectono-magmatic event sequences,stratigraphic and sedimentary features）,we suggest that the southern Beishan orogen was a rift zone during the Early Permian.
作者
许伟
徐学义
牛亚卓
陈高潮
史冀忠
魏建设
宋博
张宇轩
XU Wei;XU XueYi;NIU YaZhuo;CHEN GaoChao;SHI JiZhong;WEI JianShe;SONG Boz;ZHANG YuXuan(School of Earth Science ＆ Resources,Chang＇an University,Xi＇an 710054,China;China Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,MLR,Xi＇an Center of Geological Survey,Xi＇an 710054,China;China Geological Survey,Beijing 100037,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2018年第10期3011-3033,共23页
Acta Petrologica Sinica
基金
国家自然科学基金项目(41402195
41402097)
中国地质调查局地质调查项目(DD20160172
12120115002301-01)联合资助
