摘要
冀北赤城花岗岩主要出露于赤城县城以北的马营、云州水库、独石口和镇宁堡等地,包括马营的红墩梁、云洲水库的舍身崖、独石口的冰山梁和镇宁堡的东沟楼等四个岩体。这些花岗岩体均呈岩珠状产出,为中深成至深成侵入岩,它们主要侵位于晚古生代―红旗营子群‖的黑云斜长片麻岩之中,局部侵位于样墩片麻岩,部分岩体则为侏罗系火山岩所覆盖而呈沉积接触关系。其中,红墩梁、舍身崖和东沟楼等岩体的出露面积大小不等,为黑云二长花岗岩,具典型的花岗结构和块状构造,主要矿物成分有斜长石、钾长石、石英,次要矿物为黑云母。黑云二长花岗岩的地球化学特征显示其具较高的SiO_2含量(73.93 %～76.44 %)以及相对较低的Al__2O_3(11.56 %—12.95 %)、Na_2O (3.88 %—4.38 %)和K_2O含量(4.37 %—5.06 %),ΣREE相对较低(92.51 ppm—240.53 ppm),(La/Yb)N介于6.66与18.27之间,具明显的负Eu异常(δEu=0.15—0.38),Ce异常不明显。其fSm/Nd和εNd(t)分别为-0.31—-0.53和-13.25—-15.80,为起源于陆壳环境的钙碱性系列花岗岩类。冰山梁岩体的矿物成分有斜长石(40 %—45 %)、钾长石(30 %—35 %)、石英(10 %—15 %)和少量黑云母,在地球化学特征上,具有较低的SiO_2含量(66.31 %)以及相对较高的Al_2O_3(15.13 %)、Na_2O(4.40 %)和K_2O含量(5.51 %),ΣREE相对较高(373.95 ppm),(La/Yb)N为30.77,具不太明显的负Eu异常(δEu=0.78),Ce异常相对较明显(δCe=0.70)。其fSm/Nd和εNd(t)分别为-0.50和-12.18,为起源于陆壳环境的花岗闪长岩类。对于二长花岗岩类,马营的红墩梁岩体中的锆石具有明显的岩浆成因环带,Th/U比值为0.50～1.45,锆石SHRIMP U-Pb同位素数据表明其~(206)Pb/~(238)U平均年龄为143.9±2.2 Ma;东沟楼黑云二长花岗岩体中的锆石具有明显的岩浆成因环带,Th/U比值为0.61～0.96,锆石颗粒的SHRIMP U-Pb同位素分析数据表明其~(206)Pb/~(238)U平均年龄为138.0±2.2 Ma;云州水库的舍身崖岩体,Th/U比值为2.19,数据显示该岩体的年龄在136.0±3.3Ma左右。对于花岗闪长岩类,冰山梁岩体中的锆石也具明显的岩浆成因环带,Th/U比值较高,为1.18～2.73,锆石SHRIMP U-Pb同位素测年分析数据表明其~(206)Pb/~(238)U平均年龄为228.0±3.1 Ma。
根据获得的锆石SHRIMP年龄资料来看,以冰山粱岩体为代表的花岗闪长岩体的锆石SHRIMP U-Pb年龄为228 Ma,形成于印支期,而红墩粱、舍身崖和东沟楼等黑云二长花岗岩体的锆石SHRIMP年龄介于136Ma与144 Ma之间,平均值为143 Ma,明显形成于燕山期,这与前人认为该地区花岗岩体是海西期明显不同。
冀北赤城地区两期花岗岩体的存在,一方面暗示该地区先后经历了印支期和燕山期两次岩浆热事件,这对该地区的岩石—构造单元的大地构造属性等相关研究提供了岩石学和年代学的依据;一方面限定了冀北红旗营子群整体推覆的最晚时间应该是在228Ma之前;另一方面结合华北克拉通中生代遭受的破坏,赤城地区花岗岩的存在和年代学的研究,结合对赤城花岗岩Sm-Nd同位素的研究,不仅说明了在中生代华北克拉通北缘中段地区的岩石圈减薄作用,还为华北克拉通中生代的破坏的具体时间提供了岩石学证据,从而限定了华北克拉通的破坏始于印支期,而在燕山期达到了高潮。
The granitic plutons which emplaced in the Hongqiyingzi complex are exposed in Maying、Yunzhou reservoir、Dushikou and Zhenningpu, in the northern margin of Northern China Craton(NCC). The granitic bodies in the region of interest include Hongdunliang、Seshenya、Bingshanliang and Donggoulou et al. Some of the granitoids emplaced into the Hongqiyingzi complex which consisted of biotite plagioclase gneiss, some emplaced in Yangdun gneiss, and some other bodies were covered by upper Jurassic volcanic sedimentary rock. On the basis of the geochemical and isotopic characteristics, the granitic plutons could be classified two types. The first, calc-alkalic granitic rocks that are mainly exposed in Hongdunliang, Sheshenya, and Donggoulou area have different exposure area, but the same attitude and similar petrological feature. They mainly component of plagioclase、potassic feldspar、quartz and little blotite, with typical granitic texture and massive structure. The granites rich in SiO_2 content (73.93%～76.44%) and lower Al_2O_3 content (11.56%～12.95%)、Na_2O(3.88%～4.38%) and lower K_2O content (4.37%～5.06%), and is characterized by lower total rare-earth element content (∑REE) (92.51～240.53 ppm), (La/Yb)N between 6.66 and 18.27, and distinct negative Eu anomaly of Eu (δEu=0.15～0.38), and a weak Ce anomaly. This type present the features: the fSm/Nd average range from -0.31 to -0.53 and theεNd(t) average range from -13.25 to 15.80, so, the monzonitic granites is calc-alkalic series granites, stemming from continental crust.
The second, the granitic pluton, exposed in Bingshanliang, is granodiorite, which mainly component of plagioclase (40%～45%)、potassic feldspar (30%～35%)、quartz (10%～15%) and little blotite. The granodirite body is characterized by lower SiO_2 content (66.31%), higher Al_2O_3 (15.13%)、Na_2O (4.40%) and K_2O(5.51%), higher∑REE(373.95 ppm), the ratio of (La/Yb)N is 30.77, a weak Eu anomaly and apparent Ce anomaly(δCe=0.70). Its fSm/Nd is -0.50 and theεNd(t) is -12.18(<0), illustrates the granodiorte stemmed from continental crust too.
For the biotite monzogranite, we tested some samples from Hongdunliang and Donggoulou, and just analyses only one spot on the sample from Sheshenya for lacking of time. The zircon in the samples from Hongdunliang body (sample 072503) have apparent magmatic genetic clitellum, and its ratio of Th/U is between 0.50 and 1.45, the average SHRIMP U-Pb isotopic age ~(206)Pb/~(238)U from 14 zircons is 143.9±2.2Ma; and the zircon in the samples from Donggoulou (sample 072802) also have apparent magmatic genetic clitellum in cathodoluminescence images, and its ratio of Tu/U range from 0.61 to 0.96, the average of analytic datas of zircon U-Pb ~(206)Pb/~(238)U age is 138.0±2.2 Ma; with regard to the body exposure at Sheshenya, the zircon also have magmatic genetic clitellum in the cathodoluminescences, and based on the only one spot on the zircon, its ratio of Th/U is 2.19, the ~(206)Pb/~(238)U age is 136.0±3.3Ma. For the granodiorite, we used the body of Bingshanliang to zircon SHRIMP U-Pb dating. The zircons in samples from the Bingshanliang body (sample 072602) have apparent clitellum in the cathodouminescences, and higher ratio of Th/U (range from 1.18 to 2.73), the average of analytic datas of zircon U-Pb ~(206)Pb/~(238)U age is 228.0±3.1Ma.
Based on our study of zircon SHRIMP U-Pb dating, the ~(206)Pb/~(238)U age from the zircon of the granodiorite exposed in Bingshanliang is 228Ma, formed at Indo-Chinese epoch. On the other hand, the average of ~(206)Pb/~(238)U age, obtained from the monzonitic granite, which exposure at Hongdunliang、Sheshenya and Donggoulou, ranges from 136Ma to 144Ma, the average value is 143Ma, so, the monzonitic granites in the studying region formed at Yanshan epoch. The age of granitic plutons in Chicheng is different from the concept of other scholars who regarded the granits is formed in Haixi epoch.
The study of incorporation of granite forming and lithopheric thinning could get in touch with craton breaking The SHRIMP U-Pb age data has extraordinarily important geologic meaning. First of all, it strongly show that there are two stages of magmatic activities in Chicheng region, northern Chicheng—Chongli—Pingquan discordogenic fault, which offers exact foundation to the study of property of tectonic framework of the rock—building block in the studying area. Besides, integrating with the braking of Northern China Craton in Mesozoic era, it offers the petrologic proof for the braking of Northern China Craton, restricts that the Northern China Craton wrecked begin at Indo-Chinese epoch and attained the most intense phase at Yanshan epoch.
Finally, the study of the granites in the northern margin of Northern China Craton denotes the thinning of lithopheric, which is an extraordinary apparent about the breaking of Northern China Craton in Mesozoic era. Then, the study of the granites extends some significant petrologic proofs and some geochronologic proofs for the study of braking of Northern China Craton in Mesozoic era, which have extraordinary important geotectonic significations.
引文
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