| 其他摘要 | 华夏地块和扬子地块在中元古代晚期-新元古代早期发生碰撞、拼合而形成华南板块。在华南板块形成之后，它经历了多次强度不等的构造运动，导致了多期次的变质作用和地壳深熔作用。这些构造运动在华夏乃至整个华南地块中孕育了数量巨大的各类型花岗岩，并且在闽西北地区促使了大量伟晶岩的产生。闽西北地区伟晶岩是同期构造运动的产物，还是多期构造运动共同作用的结果？闽西北地区伟晶岩和花岗岩之间究竟存在怎样的关系，31号伟晶岩脉与附近的花岗岩是否存在“母子”关系？通过对31号伟晶岩脉及其周边花岗岩的形成时代、源区性质以及构造背景的研究，不但可以讨论伟晶岩脉和花岗岩的成因及其相互关系，还可以示踪造山作用过程并为追溯大陆演化提供依据。 本文通过对南平花岗岩和31号伟晶岩脉地球化学特征的研究，利用花岗岩全岩主量-微量元素和Sr-Nd同位素，伟晶岩铌铁矿U-Pb定年，花岗岩和伟晶岩锆石的CL图像、BSE图像、U-Pb定量、微量元素和Hf同位素组成等实验资料，获得了如下主要认识： 南平花岗岩A/CNK比值为0.88 - 1.02，属于准铝质；球粒陨石标准化REE配分模式图显示花岗岩为右倾型和Eu负异常；原始地幔标准化的微量元素蛛网图显示花岗岩相对富集高场强元素，且明显亏损Ba、Sr、Eu；在花岗岩类型地球化学分类图解中，显示南平花岗岩为A型花岗岩。 南平A型花岗岩锆石饱和温度的最低估算值为813 - 866℃，这意味着南平A型花岗岩源于干燥缺水的或难熔的源区。花岗岩主量元素组成与压力关系的图解显示，其主要形成于低压环境条件下。因此，南平A型花岗岩形成于高温-低压条件下。南平A型花岗岩的(DCe4+/DCe3+)锆石比值大多小于100，其平均值约55，δEu比值大多小于0.25，其平均值约0.21。这说明南平花岗岩岩浆源区处于还原状态，而且它在上升侵位的过程中受到较少的地壳上部物质的混染。 南平A型花岗岩锆石显示了清晰的环带结构，其Th/U比值范围为0.23 - 1.01，这些特征说明，该锆石为岩浆锆石。两个花岗岩锆石样品给出的206Pb/238U加权平均年龄约410 Ma，对应的εHf(t)值主要变化于-0.2至-3.3，其两阶段模式年龄为1286 - 1461 Ma。南平花岗岩全岩εNd(t)值变化于-1.23至-2.11，其两阶段模式年龄为1246 - 1317 Ma。南平A型花岗岩的初始岩浆是贫水的，具有较高的温度，表明其地壳物质源区是经历过脱水作用和/或熔体抽离作用的麻粒岩相火成变质岩和/或沉积变质岩。因此，我们认为南平A型花岗岩浆是由中元古代火成变质岩和少量沉积变质岩部分熔融形成，且在其演化过程中经历了分离结晶作用。 南平A型花岗岩形成于岩石圈地幔拆沉的构造背景下。在岩石圈拆沉期间，新的软流圈地幔物质上涌并填充了由于岩石圈地幔拆沉而腾空的区域。软流圈地幔的上涌会带来巨大的热量，而导致岩石圈地幔部分熔融，并形成变质核心区边缘的辉长岩体。软流圈物质上涌流也增加了地热梯度，从而使得下地壳发生熔融，并形成各种类型的花岗质岩浆。 I带是伟晶岩脉最早结晶的结构带，387 Ma可以近似代表31号伟晶岩脉侵入迪口组并开始结晶的时间，366.1 Ma代表伟晶岩Ⅱ带的结晶年龄，343Ma代表了31号伟晶岩脉Ⅲ带的结晶年龄，31号伟晶岩脉的演化时长约44Ma。 31号伟晶岩脉是通过熔融作用而不是分异结晶作用形成，其εHf(t)值主要变化于-11.5至-14.8，两阶段模式年龄为1838 - 2053 Ma，其源岩是古元古代沉积变质岩。华夏地块向北俯冲到扬子地块之下，并在410 Ma之前发生岩石圈地幔拆沉作用，软流圈物质随后上涌并带来巨大的热量，使得华夏地块古元古代沉积变质岩基底发生部分熔融。生成的熔体逐渐聚集在一起并形成岩浆房，岩浆沿着有利的构造部位向上运移、侵位、冷却形成伟晶岩脉。 从物源的角度考虑，南平A型花岗岩浆是由中元古代火成变质岩和少量沉积变质岩部分熔融形成，而31号伟晶岩脉应该是古元古代沉积变质岩部分熔融的产物。它们具有不同的物质来源，因此不可能存在成因联系。但是，二者均形成于造山后的伸展构造背景，为同一造山运动中不同源岩部分熔融的产物。; The South China Block (SCB) consists of the Yangtze Block to the northwest and the Cathaysia Block to the southeast, amalgamated during the late Mesoproterozoic - Neoproterozoic. The SCB has experienced several tectonic movements at varying degrees after its formation, resulting in multi-period metamorphism and anatexis. These tectonic movements in the Cathaysia Block, even in the SCB, leaded to a large number of various types of granite, and promoted the production of some pegmatites in the northwest Fujian province. However, the following questions remain: (1) the pegmatites in the northwest Fujian province is the product of the tectonic movement in the same period, or the result of the multi-stage tectonic movement; (2) What is the relationship between pegmatites and granites in the northwestern Fujian Province, and whether there is a "mother-child" relationship between the No.31 pegmatite vein and the nearby granites. Based on the study of the formation age, source region and tectonic setting of the No.31 pegmatite vein and its surrounding granites, we can not only discuss the genesis and interrelationship of the No.31 pegmatite vein and granites but also trace the process of orogeny and the evolution of the mainland.Through the study of the geochemical characteristics of Nanping A-type granite and No.31 pegmatite vein, this paper presents: (1) the major element contents, trance element contents and Sr-Nd isotopes of granite; (2) U-Pb dating of columbite-(Fe) from pegmatite vein; (3) CL images, BSE images, U-Pb dating, trace elements and Hf isotope from pegmatite and granite zircons. We obtained the following main understandings: The ratio of A/CNK in Nanping granite is 0.88 - 1.02, indicating that it is metaluminous. In the chondrite-normalized rare earth element diagram, Nanping granite is characterized by slightly enriched light rare earth elements and negative Eu anomalies. In the primitive mantle-normalized trace element spider diagram, Nanping granite shows a relative enrichment of high field strength elements and pronounced depletion of Ba, Sr, Eu and Ti. In the geochemical classification diagram of granite type, Nanping granite belongs to A-type granite.The zircon saturation temperatures of Nanping A-type granite is 813 - 866℃, and this temperature is likely to be an underestimation of its initial temperature. The relatively high melting or magma temperatures of the Nanping A-type granite suggest that it was derived from a refractory or dry source. The relationship between the major elements compositions of the granite and the pressure shows that it is mainly formed under low pressure environment. Therefore, Nanping A-type granite formed in high temperature - low pressure conditions. The ratio of from Nanping A-type granite is mainly less than 100, with an average of 55; the ratio of δEu is mainly less than 0.25, with an average of 0.21. This indicates that the source region of the Nanping granitic magma is in a reduced state, and it is mixed with rare material from the upper crust during the process of emplacement. Nanping A-type granite zircon shows well-developed oscillatory zoning, with a Th/U ratio of 0.23 to 1.01, which indicates that the zircon is a magmatic zircon. The 206Pb/238U weighted average age of two granite zircon samples is 410 Ma, and the corresponding εHf(t) value is mainly changed from -0.2 to -3.3, and the two-stage model age is 1286 - 1461 Ma. The εNd(t) value of the Nanping granite varies from -1.23 to -2.11, and the two-stage model age is 1246 - 1317 Ma. Its initial magma was anhydrous and had a high melting temperature, suggesting that the crustal sources had been dehydrated and/or melt depleted, and were granulitic metavolcanic rocks and/or metasedimentary rocks. Therefore, we suggest that the formation of the Nanping A-type granite might be derived from partial melting of Meso-proterozoic metavolcanic rocks with minor metasedimentary rocks, and underwent fractional crystallization during its evolution.Nanping A-type granite formed in the tectonic setting of lithosphere mantle. During the lithosphere delamination, the asthenosphere mantle material was uplifted and filled with vacated areas due to the delamination of the lithosphere mantle. The upwelling asthenosphere mantle could bring huge heat, which caused the partial melting of lithosphere mantle and formed gabbros at the edge of the metamorphic core. The heat from asthenosphere also increased the geothermal gradient, which caused the partial melting of crust and formed various types of granitic magma.Zone I is the earliest crystalline zone of the pegmatite vein, and the time of crystallization of the No.31 pegmatite vein and its emplacement into the Diou Formation is about 387 Ma. 366.1 Ma represents the crystallization age of the No.31 pegmatite zone II, 343 Ma represents the crystallization age of No.31 pegmatite zone Ⅲ, the evolution time of No.31 pegmatite vein was about 44 Ma. No.31 pegmatite vein formed by direct melting rather than fractional crystallization, its εHf(t) value is mainly changed from -11.5 to -14.8, and the two-stage model age is 1838 - 2053 Ma, and its source rock was Paleo-proterozoic metasedimentary rocks. The Cathaysia Block northward subducted to the the Yangtze Block, and underwent the lithosphere mantle delamination. Then the asthenosphere mantle material was uplifted and could bring huge heat, resulting in the partial melting of Paleo-proterozoic metasedimentary rocks in The Cathaysia Block. The generated melt gradually gathered together and formed a magma chamber, where the magma migrated upwardly along the favorable tectonic site, emplaced and cooled, and finally formed pegmatite veins. From the point of view of material source, the Nanping A-type granite magma might be derived from partial melting of Meso-proterozoic metavolcanic rocks with minor metasedimentary rocks. However, the No.31 pegmatite vein should be the products of the partial melting of Paleo-proterozoic metasedimentary rocks. They have different sources materials, so there are no genetic relationship between the Nanping A-type granite and No. 31 pegmatite vein. However, both of them are formed in the extensional tectonic setting after orogeny, and are the products of different source rocks by partial melting in the same orogeny. |
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