满洲里新右旗火山岩剖面年代学和地球化学特征

Geochronology and geochemical characteristics of volcanic rocks section in Manzhouli Xinyouqi,Inner-Mongolia

满洲里新巴尔虎右旗地区发育大面积中生代火山岩,通过典型火山岩剖面研究,本文认为该地区火山岩主要为粗面安山岩、粗面岩和流纹岩组合;剖面下、中部粗安岩和流纹岩基质^(40)Ar/^(39)Ar 坪年龄分别为162.6±0.7Ma 和162.0±0.8Ma,剖面上部粗安岩 SHRIMP 锆石 U-Pb 年龄为149.5±1.7Ma,表明满洲里新巴尔虎右旗地区火山岩形成于晚侏罗世,并可能存在两次火山喷发旋回,间隔大约10Ma。地球化学测试表明,该剖面中基性火山岩具有相似的地球化学特征,SiO_2含量介于56.23%～61.53%之间,具有较高的 Al_2O_3(15.38%～16.62%)、K_2O(2.72%～3.87%)和全碱(6.28～8.79),较低的MgO(0.91%～3.8%)和 Mg~#(0.10～0.32),为高钾钙碱性火山岩;稀土总量(∑REE)及 LREE 富集,LREE 和 HREE 分馏较强,Eu 异常不明显(δEu=0.72～0.90),富集大离子亲石元素(LILE),而亏损高场强元素(HFSE),尤其强烈亏损 Nb 和 Ta;(^(87)Sr/^(86)Sr)_i 变化范围为0.705078～0.705811,ε_(Nd)(t)值变化范围为-0.5～1.3。而剖面中部的流纹岩具有高 SiO_2(79.88%～80.46%)、相对低 MgO(0.04%)、FeO(0.26%)和 Al_2O_3(9.61%～10.17%)的特征,负 Eu 异常明显(δEu=0.32),富集 LILE而亏损 HFSE,Sr、Ti、Ba 和 P 强烈亏损,(^(87)Sr/^(86)Sr)_i 变化范围为0.710044～0.713891,ε_(Nd)(t)值变化范围为-0.9～-0.7。本区剖面火山岩地球化学特征表明,下部和上部中基性火山岩因具有相似的地球化学特征,本文推测该类火山岩浆源于受早期俯冲洋壳流体交代的地幔楔或亏损地幔,粗面岩为中基性岩浆分离结晶的产物,流纹岩包含了大量的地壳物质,其成因可能与地壳混染或大陆中下地壳重融作用有关。结合前人资料,我们认为满洲里新巴尔虎右旗晚侏罗世火山岩形成于造山后阶段,是岩石圈伸展背景下被早期流体交代的岩石圈地幔发生部分熔融的产物。该区火山岩的形成与晚侏罗世蒙古-鄂霍次克洋闭合-造山后伸展作用有关。

Mesozoic volcanic rocks section in Manzhouli Xinyouqi, Inner-Mongolia is mainly composed of trachy-andesite, trachyte and rhyolite. ^40Ar-^39Ar dating of lower section trachy-andesites and middle section rhyolites give a plateau age of 162.6 ±0.7Ma and 162.0 ±0.8 Ma, and zircon U-Pb determination of upper section trachy-andesites yields a concordant age of 149.5 ± 1.7Ma. This indicates that these volcanic rocks were formed in two gyrations of volcanic explosion in late Jurassic. The major elements of the samples show that geochemical characteristics of rocks in the lower and upper section are similar, which indicate that they formed from the same magma source. Both of them are relatively high in SiO2, Al2O3, K2O and total alkaline and low in MgO and Mg^#, belonging to the calc-alkaline series with high K contents and demonstrating total REE and LREE enrichment with insignificant Eu anomalies （δEu = 0. 72-0. 90） and intense fractionation between LREE and HREE. The primitive mantle-normalized spidergrams are characterized by LILE enrichment and HFSE depletion, especially for a significantly negative Nb-Ta anomaly. Isotopic geochemistry shows that the value of εNd （t） range from - 0.5 to 1.3, and （ ^87 Sr/^86 Sr） i from 0. 705078 to 0.713891. Rhyolites of middle rocks section are high in SiO2 and low in MgO, FeO and Al2O3, belonging to the calc-alkaline series. Rhyolites display the same REE pattern and εNd （t） （ -0.7 ～ -0.9 ） with trachy-andesite and trachyte, while Eu contents appears intense negative Eu anomalies（δEu =0.32）, and are characterized by Sr,Ti.Ba and P significant depletion and the high （^87Sr/^86Sr） i value（0. 710044 ～0. 713891 ）. The geochemical feature of rachy-andesites and trachytes suggest their origination were from a metasomatized mantle wedge or depleted mantle, which had been modified by previous subducted ocean slab. According to the correlation between the rachy-andesites and traehytes, it is inferred that the trachytes were derived from the differentiated crystallization of andesitic magmas. On the other hand, the rhyolites possess a clear ＂crustal＂ signature, possibly attributed to the crustal contamination during the magma ascent or anatexis of lower-middle crust in response to the andesitic underplating. Combining their geochemical characteristics with previous studies of this area, the late Jurassic volcanism in Manzhouli Xinyouqi was formed in the post-orogenic stage in which a rapid lithospheric extension must have accurred, resulting in the decompressional melting of the pre-existent metasomatized lithospheric mantle. It is possibly that these Mesozoic volcanic rocks in Manzhouli Xinyouqi were formed by the collision-postorogenic extension of Mongolia-Okhotsc ocean during late Jurassic.