首次对大别山北缘西段(河南省境内)中生代火山岩进行了锆石原位U-Pb同位素测年.结果表明,罗山县双桥火山岩集中区石英安山岩锆石LA-ICP-MS U/Pb年龄为133.1±1.5Ma,与大别东段中生代火山岩主体毛坦厂组的时代一致,属早白垩世,稍早于或与大别山区大规模花岗岩岩基侵位时代相同;与长江中下游地区的宁芜和庐枞盆地火山岩喷发时间同步,可能同属中国东部中生代巨量岩浆活动的重要组成部分,与太平洋板块斜向俯冲引起的大陆岩石圈重力不稳而产生的拆沉导致岩石圈强烈减薄深部动力学过程相联系.大别山西段早白垩世火山岩属高钾钙碱性系列粗安岩-英安岩-流纹岩组合.中酸性火山岩显示轻稀土元素富集、重稀土元素强烈亏损、弱或基本无负Eu异常的稀土元素配分模式以及高Sr低Yb特征,反映岩浆部分熔融源区残余有石榴石,说明当时存在加厚的成熟陆壳.高(87Sr/86Sr)i (0.707 56)、极低的εNd(t)(-20.1)和大离子亲石元素富集、高场强元素Nb、Ta明显亏 损,显示岩浆源区的壳源性质.以上特征反映大别山地区该时期岩石圈伸展程度和软流圈上涌规模远不如长江中下游同时期以幔源为主的中基性火山岩发育的宁芜和庐枞地区.%In-situ LA-ICP-MS zircon U-Pb dating of dacite from the Chenpeng Formation volcanic suite in Shuangqiao area, Luoshan County, Henan Province, demonstrates that the Mesozoic volcanic rocks from the western segment of northern Dabie Mountain were formed at ca. 133 ±1.5 Ma, being coeval with the main vol-canism in the eastern segment of the Dabie Mountain, which is represented by the Early Cretaceous Maotan-chang Formation volcanic rocks. The Mesozoic volcanism in western North Dabie is also coeval with, or slightly earlier than the large-scale intrusion of granodioritic plutons in the Dabie Mountain and the large-scale volcanism in the Lu-Zong and Ning-Wu volcanic basins along the middle and lower reaches of the Yangtze River. Thus, the results obtained by the authors suggest that the volcanic rocks in western North Dabie probably represent one of the large scale Mesozoic magmatic activities in eastern China, which has commonly been thought as results of strong lithosphere thinning and delamination caused by continental gravity instability, which was likely induced by the oblique subduction of Pacific plate. Detailed bulk-geochemistry of granodioritic rocks of the Mesozoic volcanic suite in western North Dabie exhibits REE patterns with strong LREE enrichment, HREE depletion, and weak or almost no Eu negative anomaly. These volcanic rocks also display high Sr- and low Yb-concentration, suggesting the existence of garnet, which might have been a major residual phase during partial melting. Such geochemical features imply that the magma resulted from the partial melting of a highly-thickened mature continental crust during the Mesozoic volcanism, just like that of the entire Dabie Mountain. In contrast with the vol-canics in the Ning-Wu and Lu-Zong basins and eastern Dabie Mountain, the western North Dabie volcanic rocks are characterized by somewhat higher (87Sr/86Sr); (0.707 56) and remarkably lower eNd(t) (-20.1), and more enriched LILE and depleted Nb and Ta. These geochemical features suggest that western Dabie volcanic rocks have experienced lower degree of metasomatism of mantle-derived materials than volcanic rocks in eastern China, which further suggests a westward decrease in the Mesozoic lithosphere stretching and asthenosphere up-welling.
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