【作者】 曹花花；

【导师】 许文良；

【作者基本信息】 吉林大学 ， 矿物学、岩石学、矿床学， 2010， 硕士

【摘要】 本文对珲春地区晚海西期辉长岩和闪长岩进行了锆石LA-ICP-MS U-Pb定年、Hf同位素和岩石地球化学研究,确定了它们的形成时代,并讨论了岩石成因以及研究区二叠纪的构造演化。辉长岩和闪长岩中的锆石均呈自形-半自形,发育典型的岩浆振荡生长环带,结合高的Th/U比值(0.26~1.22),暗示其为岩浆成因。测年结果表明:辉长岩的形成时代为早二叠世(282Ma);闪长岩的形成时代为晚二叠世(255Ma),同时闪长岩中存在早二叠世(279Ma)的捕获锆石。辉长岩属于低钾拉斑系列,具有较低的稀土元素(REE)丰度,较平坦的REE配分形式和较弱的Eu正异常,明显亏损高场强元素(Nb、Ta、Ti),显示岛弧玄武岩的地球化学属性,锆石εHf (t)值介于+7.63～+14.6之间,显示辉长岩岩浆起源于亏损的岩石圈地幔。闪长岩属于中钾钙碱性系列,具有明显偏高的REE丰度,较弱的Eu正异常,以及高场强元素Nb、Ta、Ti的强烈亏损,显示活动陆缘岩浆岩的地球化学特征;闪长岩中结晶锆石的εHf (t)值介于+11.22～+14.17之间,其Hf模式年龄为424~692Ma,表明闪长岩岩浆起源于从亏损地幔分异出来的早古生代或新元古代新生下地壳的部分熔融。早二叠世辉长岩至晚二叠世闪长岩地球化学属性的变异揭示兴凯地块与和龙地块(华北板块北缘东段)之间曾存在古亚洲洋向兴凯地块西南缘的俯冲作用和消亡的演化历史。更多还原

【Abstract】 The Yanbian fold belt is located at the junction of the Jiamusi, Khanka and Longgang massifs, i.e., the easternmost segment of the Hinggan-Mongolia Orogenic Belt (XMOB). The region mainly experienced collision and amalgamation of the Jiamusi, Khanka and Longgang massifs during the Paleozoic. For a long time, the Late Paleozoic tectonic nature between the Khanka and Longgang massifs has always been a controversial issue. At present, two different opinions are presented based on the closure time of the Paleo-Asian Ocean, the late Paleozoic sedimentary strata, and paleontological evidence. One is that the Khanka and Longgang massifs were separated by an aulacogen in the Permian; the other is that the closure of the Paleo-Asian Ocean between the Khanka and Longgang massifs happened in the Permian. In the latter view, some scholars consider that the Paleo-Asian Ocean subducted beneath the southwestern edge of the Khanka Massif. Then, whether did the subduction between the Khanka and Longgang massifs happen or not? How did the Paleo-Asian Ocean crust subduct beneath the Khanka Massif or the Longgang Massif? The above-mentioned problems have been unsolved due to the lack of detailed dating and geochemical data for the magmatic rocks in the region. However, a voluminous Late Hercynian intermediate-basic rocks, occurred in the Hunchun area, provide insight to solve the above-mentioned problems. Therefore, this paper presents zircon U-Pb dating, Hf isotope, and geochemical data for the Late Hercynian gabbros and diorites in Hunchun area to constrain the tectonic evolution between the Khanka and Longgang massifs in the Late Paleozoic.1. Formation time of gabbro and diorite LA-ICP-MS zircon U–Pb dating results show that zircons from the gabbro in Hunchun Area yield a weighted mean 206Pb/238U age of 282±2 Ma, indicating that the gabbro formed in the Early Permian, that the magmatic zircons from the diorite yield a weighted mean 206Pb/238U age of 255±3 Ma, indicating that the diorite formed in the Late Permian, and that the captured zircons from the diorite yield a weighted mean 206Pb/238U age of 279±4 Ma, which is consistent with the formation time of the gabbro under the uncertainty.2. Geochemistry and zircon Hf isotopes of the gabbros and dioritesThe gabbros in Hunchun area have SiO2 = 46.75-47.54%, Mg# = 61-70, high Al and low Ti (Al2O3 = 19.99-20.87%, TiO2 = 0.37-0.41%), belonging to the subalkaline low-K tholeiitic series. Chondrite-normalized rare earth element (REE) patterns indicate that the gabbros are characterized by relatively enrichment in LREEs and depletion in HREEs, and weak positive Eu anomalies. Meanwhile, the gabbros display an enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, and K), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), and strong positive Sr anomalies, similar chemically to island-arc tholeiites. 176Hf/ 177Hf ratios andεHf(t) of selected zircons from the gabbro vary from 0.282813 to 0.283015 and from +7.63 to +14.61, respectively.The diorites have relatively high SiO2 contents (SiO2 = 54.56-61.23%), lower Mg (Mg# = 37-39), high alkali contents (Na2O+K2O = 4.74-5.19%), belonging to the subalkaline middle-K calc-alkaline series. Compared with the gabbros, the diorites have the similar chondrite-normalized REE patterns and primitive mantle (PM)-normalized trace element patterns, e.g., enrichment in LREEs and large ion lithophile elements, depletion in HREEs and high field strength elements. However, the diorites have obviously higher REE and other trace element abundances, and weak negative Eu anomalies, similar to the igneous rocks from the active continental margin setting. 176Hf/ 177Hf ratios andεHf(t) of zircons with age of 255 Ma from the diorites are between 0.282940 and 0.283040 and between +11.22 and +14.17,respectively. Their Hf model ages range from 424 Ma to 692 Ma.3. Magma sourceThe gabbros in the Hunchun area are poor in SiO2 (46.75-47.54%), rich in MgO (7.56- 8.07%), high Mg# (61-70), and high Cr (201-229 ug/g) contents, suggesting that their primary magma could be derived from partial melting of peridotite mantle.The depletion in Nb, Ta and Ti of the gabbros could exclude the possibility that the gabbro magma could be derived from partial melting of the lithospheric mantle metasomated by the subducted slab-derived melt at the mantle wedge. Therefore, we propose that the primary gabbro magmas should be derived from partial melting of mantle wedge peridotite metasomatized by subduction- related fluid, which is consistent with their high Al and low-K tholeiitic geochemical features and is also supported by their low Zr/Y ratios.εHf(t) values of zircons from the gabbros vary from +7.63 to +14.61, suggesting that primary magma should be derived from partial melting of a depleted lithospheric mantle. Taken together, the gabbro magmas should originate from partial melting of the depleted mantle wedge metasomatized by the subducted slab-derived fluid. Combined with their similarity to volcanic arc low-K tholeiites, it is suggested that the gabbros could form in island arc setting close to trench.The Wudaogou diorites are characterized by enrichment in SiO2 and alkali, as well as depletion of MgO, Cr (3.66 ug/g ~7.29 ug/g) and Ni (1.91 ug/g ~ 5.35 ug/g), implying that they could not be the mantle-derived primary magma. TheεHf(t) values and Hf model ages of zircons from the diorite range from +11.22 to +14.17 and from 424 to 692 Ma, respectively, suggesting that the primary magma for the diorites could be derived from partial melting of the Early Paleozoic and/or Neoproterozoic accreted lower crust.4. Tectonic implicationsThe gabbros in Hunchun area show the geochemical features of the igneous rocks from the island-arc setting, suggestting that the Paleo-Asian oceanic plate subducted beneath the the Khanka Massif in the Early Permian. The diorites, together with the coeval tonalitic rocks from the Xiaoxinancha area, are chemically similar to the igneous rocks from the active continental margin setting, implying that an active continental margin setting could exist in the region in the late Permian. The gabbros and diorites are both located in the western margin of the Khanka Massif, and fromed in the Early and Late Permian, respectively. Spatial and temporal distribution of gabbros and diorites implies that the collision and amalgamation of the arc and continent happened in the region between the Early Permian and Late Permian.更多还原