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在《湖北省区域地质志》修编过程中,将原花山群北部沿着土门—小阜—园潭一带分布的一套火山岩夹少量碎屑岩组合解体出来,新定义为土门岩组,代表青白口纪扬子北缘的一套岛弧火山—碎屑岩建造,标准剖面在李家塆—土门公路边。土门岩组以基性玄武岩为主,中性安山岩和酸性英安岩、流纹岩次之,局部夹有少量的火山碎屑岩和粉砂岩,受后期构造活动影响产生一定程度的变形和轻微的变质作用。土门岩组火山岩SiO_2的含量为50.47%~81.57%,由基性、中性、酸性三个端元组成,大部分表现为钙碱性系列的特点,少量表现为拉斑玄武岩系列特点,轻稀土元素(LREE)富集,具有“右倾”的稀土配分模式以及一定程度的负Eu异常,富集大离子亲石元素(LILE)RB、Ba、K、Pb,亏损高场强元素(HFSE)Nb、Ta、Ti等,这些特征与典型的岛弧火山岩基本相符。分别对土门和园潭酸性火山岩进行LA-ICP-MS锆石U-Pb测年,获得土门英安岩成岩年龄为841 Ma,园潭酸性火山岩的成岩年龄为(827.9±5.9)Ma(MSWD=0.57),二者同属青白口纪,都代表土门岩组火山岩的形成时间。土门岩组岛弧火山岩可能是扬子北缘晋宁期洋陆俯冲消减的产物,结合已经获得的三里岗岛弧中酸性侵入岩858~876 Ma和基性辉长岩871 Ma年代学资料,说明扬子北缘大洪山地区至少在876~828 Ma发生了一定规模的洋—陆俯冲消减作用。
In the process of revising the “Hubei Regional Geology”, a small volcanic rock assemblage distributed along the Tumen-Xiaofu-Yuantan area along the northern part of the original Huashan Group was disintegrated and newly defined as Tumen Group , On behalf of the northern margin of Qingbaikoujiyangzi Island arc volcano - clastic rock construction, the standard profile in Lijiajie - Tumen Road side. The Tumen Formation is dominated by basaltic basalts, neutral andesite and acidic dacite, followed by rhyolite, with a small amount of volcaniclastic and siltstone sandwiched locally, resulting in some degree of deformation under the influence of later tectonic activities and Mild metamorphism. The content of SiO_2 in volcanic rocks of Tumen Formation is 50.47% ~ 81.57%, which is composed of three terminal elements, basic, neutral and acidic. Most of them show the characteristics of calc-alkaline series. (LREE) enrichment, REE pattern with “right tilt” and a certain degree of negative Eu anomaly enrichment of LILE, RB, Ba, K, Pb, depleted high field strength elements HFSE) Nb, Ta, Ti, etc. These features are basically consistent with the typical island arc volcanic rocks. LA-ICP-MS zircon U-Pb dating of the Tumen and Yuantan acidic volcanic rocks respectively, the age of the Tumen dacite is 841 Ma, and the age of the Yuantan acidic volcanic rock is (827.9 ± 5.9) Ma MSWD = 0.57), both of which belong to Qingbaikikou, represent the formation time of volcanic rocks of Tumen Formation. The Tumen-Shi island arc volcanic rocks may be the product of the subduction of the Yangjin subduction of the Jinning Period in the northern margin of the Yangtze. Combined with the 858-876 Ma acid-derived intrusive rocks in the Sanliang Island arc and the 871 Ma geochronological data of the basic gabbro, Indicating that a certain amount of ocean-continent subduction reduction occurred in the Dahongshan area on the northern margin of the Yangtze at least from 876 to 828 Ma.