摘要:
文章系首次报道大座子山铜多金属矿火山岩特征。矿区位于华北地台北缘,西拉木伦河深大断裂和少郎河深大断裂之间,矿区流纹岩、粗面岩与成矿密切相关。流纹岩SiO2含量74.93%~76.36%,Al2O3含量11.22%~11.70%,较为富碱,K2O/Na2O为1.29~1.54;粗面岩SiO2含量65.52%~68.04%,Al2O3含量15.05%~16.38%,富碱,K2O/Na2O为0.85~1.64。两种岩石均属于准铝-弱过铝系列。流纹岩属高钾钙碱性系列;粗面岩属高钾钙碱性-钾玄岩系列。两种火山岩稀土配分整体呈右倾趋势,流纹岩强烈Eu负异常,粗面岩Eu呈弱正异常特征。除Ba、Sr、Ti等明显低于粗面岩外,流纹岩其他微量元素含量均与粗面岩相当,呈富集大离子亲石元素(LILE),亏损高场强元素(HFSE)特征。两种岩石具有类似的(87Sr/86Sr)i和(143Nd/144Nd)i初始比值,分别介于0.7046~0.7063和0.5119~0.5122(t=155Ma),εNd(t)为-4.24~-10.37,同为存留于中元古界下地壳的斜长角闪岩类经重熔作用而形成。大座子山流纹岩、粗面岩形成的构造环境是造山期后伸展条件的板内环境,即在晚侏罗世(150~160Ma)受到了蒙古-鄂霍次克海"剪刀式"闭合造山后的伸展条件和岩石圈减薄作用的影响,由存留于中元古界下地壳的斜长角闪岩类熔融而形成的一套高钾钙碱性-钾玄岩系列的火山岩。
Abstract:
This paper first reports the characteristics of volcanic rocks in the Dazuozishan copper polymetallic deposit of Inner Mongolia. The deposit is located on the northern margin of the North China Platform, between Sila Mulun River great fault and Shaolang River great fault. The rhyolites and trachytes in the deposit are closely related to the mineralization. The rhyolites contain 74.93%-76.36 % of SiO2 and 11.22%-11.70% of Al2O3. The rhyolites exhibit slightly alkali-rich form and the ratio between K2O and Na2O is approximately between 1.29 and 1.54. Meanwhile, the trachytes contain 65.52%-68.04% of SiO2 and 15.05%-16.38% of Al2O3. They are of the alkali-rich type and the ratio between K2O and Na2O is approximately between 0.85 and 1.64. Both of the two rocks belong to the metaluminous-weak peraluminous series. Meanwhile, the rhyolites belong to the high potassium calc alkali series. The trachytes, on the other hand, belong to the high potassium calc alkali ~ shoshonite series. The rare earth element (REE) patterns of these two volcanic rocks are generally rightly-inclined. The rhyolites show strong Eu negative anomaly, while the trachytes show weak Eu positive anomaly. Except the elements of Ba, Sr and Ti whose values are notably lower in rhyolites than in trachytes, the trace elements in these two rocks are comparable in composition. In addition, they are both enriched in LILE and are depleted in HFSE. These two rocks have similar initial ratios of (87Sr/86Sr)i and (143Nd/144Nd)i. The ratio of (87Sr/86Sr)i is approximately between 0.7046 and 0.7063, while the ratio of (143Nd/144Nd)i is between 0.5119 and 0.5122 (t=155 Ma) , and εNd is in the range of -4.24 to-10.37. These two rocks are the products of the anatexis of plagioclase amphibolite in the middle Proterozoic lower crust. The environment for the formation of the rhyolites and the trachytes in the Dazuozishan ore district is the intraplate environment of post-orogenic extension. In other words, under the condition of the post-orogenic extension of Mongolia-Okhotsk scissor-type closed orogeny and lithospheric thinning in late Jurassic (150-160 Ma), the anatexis of plagioclase amphibolite in the middle Proterozoic lower crust formed the volcanic rocks of metaluminous-weakly peraluminous series.
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