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1.
俯冲型和碰撞型含矿斑岩地球化学组成的差异   总被引：2，自引：2，他引：0  
Cu-Mo-Au含矿斑岩不仅可以形成于与洋壳俯冲相联系的弧环境,而且也产于碰撞造山带内。通过对比俯冲型和碰撞型含矿斑岩的地球化学特征,发现它们特别在微量元素上具有较大差别,暗示它们有着不同的物源区组成或形成机制。同冈底斯带碰撞型含矿斑岩相比,太平洋东岸俯冲型含矿斑岩有着明显高的HREE和Y含量,低的Sr/Y、(La/Yb)N以及(Dy/Yb)N比值,表明其物质源区不含或含有少量的石榴子石并可能以角闪石组成为主。统计发现这些俯冲型含矿斑岩部分样品具有埃达克岩地球化学特征,但大部分样品却显示出具有与正常岛弧系列火山岩相似的特征,它们很可能是板片释放流体交代地幔楔形成的熔体并在后期经历MASH过程的产物。冈底斯带碰撞型含矿斑岩具有典型埃达克岩地球化学特征,指示其形成条件达到了石榴子石相变,可能形成于增厚的下地壳,其物质源区很可能与前期的洋壳俯冲有着密切的联系。普朗-雪鸡坪含矿斑岩具有与俯冲型含矿斑岩十分相似的地球化学特征,它们有可能是西向俯冲的甘孜-理塘洋发生断离,进而诱发前期俯冲流体交代的富集地幔楔发生部分熔融的产物,而并非是俯冲洋壳直接发生部分熔融的产物。  相似文献   

2.
<正>The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However,their exact source and peterogenesis are still debated.~(40)Ar-~(39)Ar and LAM-ICPMS zircon U-Pb isotopic dating confirm that these rocks erupted in Eocene.In addition,the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios,similar with the adakite derived from partial melting of oceanic crust.They can be further classified as high Mg~#(Mg~#=48-57) and low Mg~# (Mg~#=33-42) subtypes.The Nadingcuo adakitic rocks have relatively low(~(87)Sr/~(86)Sr)_i and highε_(Nd)(t), showing a trend of similarity to the Dongcuo ophiolite present in the Bangong-Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong-Nujiang Ocean with 10%-20%crustal material of Lhasa terrane.On these bases we suggest that the low Mg~# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment,and the high Mg~# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge.Therefore,the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India-Asia during Cenozoic.  相似文献   

3.
Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj–Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO2, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65–70) and high contents of Cr (>300 ppm), Ni (>177 ppm), and TiO2 (>1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their 87Sr/86Sr values range from 0.7049 to 0.7053 and their ?0Nd values lie between –0.2 and 1.1. The small negative values of ?0Nd indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere; furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.  相似文献   

4.
《International Geology Review》2012,54(16):2083-2095
Early Eocene adakitic volcanic and granitoid rocks are widespread in the Eastern Pontides of NE Turkey, providing significant constraints for the early Cenozoic tectonomagmatic evolution of the region. These adakitic rock units exhibit relatively high Sr/Y and La/Yb ratios, but low Y and Yb values, similar to modern adakites generated by partial fusion of a subducted oceanic slab. They also have high K2O and low MgO contents, and show moderately enriched ISr and low ?Nd(t) isotopic signatures. Our trace element modelling suggests that these adakitic magmas were generated from partial melting at low pressures of a garnet-bearing amphibolitic source in the continental lower crust. This lower crustal melting resulted from slab break off-induced asthenospheric upwelling and related magmatic underplating beneath the Eastern Pontides. We interpret this melting event and the adakitic magmatic activity as a syn- to post-collisional process involving early Cenozoic collision of the Pontide and Anatolide–Tauride continental blocks. The geochemical and tectonic constraints presented here indicate that early Eocene adakitic magmatism in the Eastern Pontides did not result from partial fusion of a subducted oceanic slab, but instead represent continental-type adakite formation.  相似文献   

5.
We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dyn/Ybn and Lan/Ybn. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial 87Sr/86Sr and 206Pb/204Pb ratios, and the highest 144Nd/143Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.  相似文献   

6.
To date, few adakitic rocks have been reported in direct association with contemporary intra-continental extensional structures, which has cast doubt on genetic models involving partial melting of the lower crust. This study presents Early Cretaceous (143-129 Ma, new Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages) adakitic granites, which are directly associated with a contemporary metamorphic core complex (i.e., the Northern Dabie Complex in the Dabie area). These granites exhibit relatively high Sr contents, negligible to positive Eu and Sr anomalies, high La/Yb and Sr/Y ratios, but very low Yb and Y contents, similar to subducted oceanic crust-derived adakites. They are also characterized, however, by very low MgO or Mg# and Ni values, and Nd-Sr isotope compositions (εNd(t) = −14.6 to −19.4 and (87Sr/86Sr)i = 0.7067-0.7087) similar to Triassic continent-derived eclogites subducted in the Dabie-Sulu Orogen. Additionally, late granitic dikes in the adakitic intrusions exhibit low Sr contents, clearly negative Eu and Sr anomalies, low La/Yb and Sr/Y ratios, but relatively high Yb and Y contents, similar to 118-105 Ma granites in the Northern Dabie Complex. Based on composition and geochronology data of Neoproterozoic amphibolites and orthogneisses, Triassic high- to ultra-high pressure metamorphic rocks, and Early Cretaceous mafic-ultramafic intrusive rocks, and the constraints provided by experimental melt data for tonalites, metabasaltic rocks and eclogites, we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile-bearing eclogitic lower crust as a consequence of Triassic-Middle Jurassic subduction and thrusting. The late dikes probably originated from plagioclase-bearing intermediate granulites. Moreover, we suggest that late Mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of Early Cretaceous mafic-ultramafic intrusive rocks in the Dabie area, and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite. Asthenospheric upwelling in response to post-collisional delamination of lithospheric mantle was likely to have provided the heat source for the Cretaceous magmatism.  相似文献   

7.
内蒙古苏尼特左旗巴音乌拉二叠纪花岗闪长岩、石英闪长岩分布于二连-贺根山蛇绿岩带的北侧,1∶20万区域地质调查将其归于华力西晚期侵入体(γδ4(3)).经研究,该岩体具高铝(Al2O3=13.69%～16.48%)、富钠(Na2O＝3.53%～4.58%)、贫钾(K2O＝2.12%～4.10%)、高锶(Sr＝425.1×10-6～645.2×10-6)的特点,SiO2=62.13%～72.87%,Y＜18×10-6(7.23×10-6～14.29×10-6),δEu＝0.71～0.83,轻重稀土元素分馏强烈,稀土元素配分曲线为右倾斜型,铕异常不明显,为典型的埃达克质岩.在微量元素原始地幔标准化蛛网图中具明显Sr高峰和Nb低谷,有与埃达克岩一致的曲线,与岛弧安山岩-英安岩-流纹岩及大陆弧安山岩-英安岩-流纹岩有明显差异.花岗闪长岩锆石U-Pb年龄为256.1±0.9 Ma,为早二叠世岩浆活动的产物.Mg#值很高(＞50),表明晚古生代洋壳向北俯冲时在75～85 km深度(角闪岩相与榴辉岩相过渡相)发生部分熔融形成埃达克质熔体,侵入地壳形成埃达克岩.  相似文献   

8.
俯冲带复杂的壳幔相互作用   总被引：15，自引：0，他引：15  
俯冲带除俯冲板片脱水形成的富大离子亲石元素流体、交代地幔楔形成的岛弧钙碱性玄武岩安山岩-英安岩-流纹岩及相应侵入岩组合外，还存在由俯冲扳片熔融形成的埃达克质熔体交代地慢楔形成的埃达克岩-富铌玄武岩-富镁安山岩组合，从而构成了俯冲带的流体交代与熔体交代两大类壳慢相互作用体系及相应的岩石组合。熔体交代作用的显著特点是Mg、高场强元素Nb、Ti、P等含量增加，Nd／Sr值增高，而Si、K、Na及La／Yb降低。洋壳板片或洋脊俯冲、玄武质岩浆底侵使地壳增厚，或板片断离、撕裂等作用均可产生埃达克质熔体并随之产生熔体交代作用。流体和熔体与地幔橄揽岩的相互作用构成了俯冲带复杂的地球化学体系。  相似文献   

9.
Geochemistry of Adakites from the Philippines: Constraints on Their Origins   总被引：1，自引：0，他引：1  
Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum-mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post-collision setting.
All these samples show a significant depletion in Y and HREE with respect to their "normal" calc-alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these "typical adakites" is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed "intermediate adakites", display major and trace element chemical features intermediate between those of the former group and those of normal calc-alkaline lavas. We show that magma mixing between slab-derived adakitic magmas and mafic mantle-derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.  相似文献   

10.
《International Geology Review》2012,54(13):1755-1771
The tectonic setting of the West Qinling orogenic belt (QOB) during the Middle–Late Triassic remains a subject of debate. Petrogenesis of adakitic granodiorite plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region. The Tadong adakitic granodiorite pluton in the western part of the West QOB is an important element of this system. Its petrogenesis can place severe constraints on the nature of the lower continental crust and mantle dynamics during the formation of the orogenic belt. U–Pb dates obtained through zircon laser-ablation inductively coupled mass spectrometry indicate that the Tadong pluton was emplaced at 220.2 ± 2.5 Ma, coeval with abundant magmatic rocks in the region. This indicates a prominent magmatic event in the western part of West Qinling during the Late Triassic. Geochemically the granodiorites are metaluminous to peraluminous high-K calc-alkalic and characterized by relatively high SiO2 (63.84–67.91 wt.%), Al2O3 (15.39–16.54 wt.%), and Sr (435.08–521.64 ppm), and low MgO (1.16–1.88 wt.%; Mg# = 38–46), Y (5.49–8.84 ppm) and Yb (0.34–0.91 ppm) contents, variable Eu anomalies (Eu/Eu* = 0.87–1.1), and high Sr/Y (51.72–84.45) ratios. These are compositional features of adakites that are commonly assumed to have been produced through partial melting of subducted oceanic basalt. In addition, the adakitic rocks are relatively enriched in light rare earth elements, large ion lithophile elements (Rb, Ba, Sr, Th, and K), and depleted in high field strength elements. However, petrological, geochronological, and geochemical characteristics indicate that the adakitic rocks were most likely formed by partial melting of a thickened mafic lower crust. Therefore, we suggest that the Tadong adakitic granodiorites were produced in a syn-collisional regime and associated with asthenospheric upwelling triggered by slab break-off or gravitational instability. This mechanism was responsible for generating the Late Triassic magmatism of West Qinling.  相似文献   

11.
埃达克岩是一种新型的火成岩(Sr/Y值≥20),形成于环太平洋带的大洋岛弧、大陆边缘造山带和陆缘火山弧环境,依据REE配分模式可将其划分为两种成因类型:大洋型(O-型)埃达克岩和大陆型(C-型)埃达克岩。西南太平洋带是世界上新生代埃达克岩和类埃达克岩广泛分布的地区之一。这些中酸性岩浆岩广泛分布于东南亚地区的菲律宾群岛、苏拉威西和加里曼丹中部、印度尼西亚几内亚岛和巴布亚新几内亚至所罗门群岛一带,零星见于班达岛弧、苏门答腊和西爪哇等地。研究结果表明:不同成因类型的埃达克岩具有不同的含矿性,反映各自来源于不同的岩浆岩源区。无论在西南太平洋带还是东太平洋带(智利),C-型埃达克岩(La/Yb值≥12)是俯冲板块的部分熔融作用叠加岩浆上侵过程中MASH(熔融-混染-储存-均一化)和AFC(混染-分异-结晶)作用的产物,与世界级斑岩铜-金矿床共生;而O-型埃达克岩(La/Yb比值≤12)则与俯冲的海洋平缓板块部分熔融作用有关,在西南太平洋带主要与浅成热泉金矿带和喷气型矿床有成因联系。  相似文献   

12.
Orthopyroxene-bearing granodiorite (sometimes referred to as ‘charnockite’) with an adakitic affinity is a rare type of granitoid. It is generally accepted that the stabilization of orthopyroxene in igneous charnockites essentially requires low aH2O and/or high temperatures in a closed system. However, orthopyroxene can be an antecryst in a trans-crustal magmatic system. In this regard, orthopyroxene-bearing granitoids are somewhat analogous to pseudo-charnockites, where the orthopyroxene stems from a mafic reservoir. On the other hand, the source compositions of continental adakites can vary, which is often ignored in the interpretation of their contribution to the adakitic geochemical signature. In this study, we have investigated a rare orthopyroxene-bearing felsic pluton from the Zhuyuan area of West Qinling, Central China. The Zhuyuan pluton was emplaced in the Middle–Late Triassic (222–217 Ma) and is mainly composed of metaluminous to weakly peraluminous granodiorites belonging to the high-K calc-alkaline series. Moreover, they are characterized by high Mg# values (49.7–60.9) and Sr contents (471–697 ppm), low Y (12.2–15.4 ppm) and Yb (1.03–1.24 ppm) contents, high Sr/Y (33.2–46.2) and (La/Yb)N (15.3–21.4) ratios, and weakly negative Eu anomalies (Eu/Eu* = 0.78–0.89). The Zhuyuan adakitic granodiorites exhibit fairly limited Sr–Nd–Pb isotopic ratios and variable zircon initial Hf isotopes, indicating a major contribution from the Neoproterozoic basement of the Qinling Orogenic Belt. There is no evidence of any formation through high-pressure magmatic processes, and we propose that the adakitic signature of the Zhuyuan pluton could have been inherited from its source rocks (i.e., from the Neoproterozoic basement). The orthopyroxenes in the Zhuyuan granodiorites display poikilitic textures with high MgO, NiO and Cr2O3 contents, indicating that they have an antecrystic origin. Studies of regional tectonic evolution have shown that the Zhuyuan granodiorites formed during the tearing stage of the A'nimaque–Mianlue oceanic subduction slab. Therefore, this study emphasizes the effect of source inheritance on the formation of pseudo-charnockite with an adakitic signature.  相似文献   

13.
李世超  张凌宇  李鹏川  施璐  郑常青 《地球科学》2017,42(12):2117-2128
为了研究大兴安岭中段地区早三叠世的构造背景问题,以下三叠统哈达陶勒盖组火山岩为研究对象,通过野外观测和X射线荧光光谱分析法、等离子质谱法分别对样品的主量、微量元素进行了测试.结果显示哈达陶勒盖组火山岩是一套中酸性钙碱系列岩石组合,具有高Sr、低Yb与Y和高Sr/Y的特点,为钠质（O型）埃达克岩.结合区域资料认为,古亚洲洋于早三叠世时期闭合的碰撞应力使贺根山-嫩江-黑河构造带活化,推动构造带内年轻地壳由南向北俯冲,进而推动古生代残留洋壳向更深的地幔插入,在此过程中加热并熔融、喷出地表形成哈达陶勒盖组埃达克岩,并在山间盆地沉积了湖相碎屑堆积-老龙头组.   相似文献   

14.
本文以贺根山缝合带呼都格奥长花岗岩体为研究对象,通过野外地质调查和岩石学、地球化学、锆石U-Pb年代学研究,讨论岩石成因、构造环境、TTG岩浆事件及古亚洲洋俯冲消亡过程。岩石地球化学研究表明,呼都格岩体富硅(SiO2=66.27%~71.59%)、高铝(Al2O3=15.23%~15.94%)、富钠(Na2O=4.13%~6.59%)、低钾(K2O=1.72%~2.53%),相对高锶(Sr=196.60×10-6~465.40×10-6)、低钇(Y=5.70×10-6~12.63×10-6),富集Ba、Sr等大离子亲石元素和LREE,亏损Nb、Ta、Ti、P等高场强元素和HREE,无明显Eu异常。岩石学和岩石地球化学特征表明,呼都格岩体属于以奥长花岗岩为主的英云闪长岩-奥长花岗岩-花岗闪长岩TTG岩石组合。这套TTG组合除Sr、Mg、Ni和Cr含量相对较低之外,与高Si埃达克岩的地球化学特征相类似,形成于大洋俯冲带岛弧环境,可能为俯冲洋壳脱水熔融成因。锆石LA-ICP-MS U-Pb测年获得两组年龄为306.3±1.9Ma和315.5±1.9Ma,表明该岩体侵位于晚石炭世,反映了贺根山缝合带晚石炭世大洋俯冲带TTG岩浆事件。结合其与梅劳特乌拉-高力罕蛇绿岩-TTG岩带前弧玄武岩、高镁安山岩/高镁闪长岩、埃达克岩、TTG、富铌弧玄武岩/辉长岩的岩石构造组合,认为古亚洲洋二连-贺根山洋盆在晚石炭世可能处于洋壳俯冲消减、TTG岩浆活动和新生陆壳生长洋陆转换过程中。  相似文献   

15.
New geochemical and isotopic data for post-collisional Early Eocene and Late Miocene adakitic rocks from the eastern part of the Sakarya Zone, Turkey, indicate that slab and lower crustal melting, respectively, played key roles in the petrogenesis of these rocks. The Early Eocene Yoncal?k dacite (54.4 Ma) exhibits high Sr/Y and La/Yb ratios, low Y and HREE concentrations, moderate Mg# (44–65), and relatively high εNd and low ISr values, similar to adakites formed by slab melting associated with subduction. Geochemical composition of the Yoncal?k dacite cannot be explained by simple crystal fractionation and/or crustal contamination of andesitic parent magma, but is consistent with the participation of different proportions of melts derived from subducted basalt and sediments. Sr/Y correlates horizontally with Rb/Y, and Pb/Nd correlates vertically with Nd isotopic composition, indicating that Sr and Pb budgets are strongly controlled by melt addition from the subducting slab, whereas positive correlations between Th/Nd and Pb/Nd, and Rb/Y and Nb/Y point to some contribution of sediment melt. In addition to low concentrations of heavy rare earth elements (~2–3 times chondrite), a systematic decrease in their concentrations and Nb/Ta ratios with increasing SiO2 contents suggests that slab partial melting occurred in the garnet stability field and that these elements were mobilized by fluid flux. These geochemical and isotopic signatures are best explained by slab breakoff and fusion shortly after the initiation of collision. Although the Late Micone Tavda?? rhyolite (8.75 Ma) has some geochemical features identical to adakites, such as high Sr/Y and La/Yb ratios, low Y and HREE concentrations, other requirements, such as sodic andesite and/or dacite with relatively high MgO and Mg# (>50), relatively high Ni and Cr, low K2O/Na2O (<0.4), high Sr (>400 ppm), for slab-derived adakites are not provided. It is sodic in composition and shows no traces of fractionation from dacitic parent magma. Low Nd and high Sr isotope ratios suggest derivation by partial fusion of calc-alkaline, juvenile crust with high Sr/Y and La/Yb ratios.  相似文献   

16.
内蒙古达茂旗北部奥陶纪埃达克岩类的识别及其意义   总被引：19，自引：0，他引：19  
内蒙古达茂旗北部奥陶纪侵入岩分布于槽台边界断裂(乌兰布拉格—哥舍深大断裂)以北,岩石类型主要有闪长岩、石英闪长岩、英云闪长岩和花岗闪长岩。该套侵入岩具高w(Sr)、低w(Y)和w(Yb)的特征,具高的w(Sr)/w(Y)和w(La)/w(Yb)比值;轻稀土富集,重稀土亏损,具正铕异常或微弱的负铕异常;微量元素蛛网图中具明显的Sr峰和Nb、Ti低谷,与典型的埃达克岩(adakite)特征一致。通过与典型adakite、太古代TTD(G)及岛弧和大陆边缘弧ADR的对比,表明本区奥陶纪侵入岩以具轻、重稀土强烈分馏型而与岛弧和大陆边缘弧ADR区别明显,而与adakite和TTD(G)相似,同时又以具明显的Sr正异常而区别于太古代的TTD(G)。地球物理、重力及航磁资料显示槽台边界断裂两侧有完全不同的地壳结构。北侧可能是在晚古生代早期拼贴到华北陆块的一个小微地块,拼贴过程中二者之间的加里东洋壳向微地块之下俯冲消减,俯冲板片在榴辉岩相或角闪岩相—榴辉岩相过渡相发生部分熔融形成adakite岩浆,熔融残留物主要为石榴石。  相似文献   

17.
Adakitic rocks and related Cu–Au mineralization are widespread along eastern Jiangnan Orogen in South China. Previous studies have mainly concentrated on those in the Dexing area in northeastern Jiangxi Province, but information is lacking on the genesis and setting of those in northwestern Zhejiang Province. The Jiande copper deposit is located in the suture zone between the Yangtze and Cathaysia blocks of South China. This paper presents systematic LA–ICP–MS zircon U–Pb dating and element and Sr–Nd–Hf isotopic data of the Jiande granodiorite porphyry. Zircon dating showed that the Jiande granodiorite porphyry was produced during the Middle Jurassic (ca. 161 Ma). The Jiande granodiorite porphyry is characterized by adakitic geochemical affinities with high Sr/Y and LaN/YbN ratios but low Y and Yb contents. The absence of a negative Eu anomaly, extreme depletion in Y and Yb, relatively low MgO contents, and relatively high 207Pb/204Pb ratios, indicated that the Jiande granodiorite porphyry was likely derived from partial melting of the thickened lower continental crust. In addition, the Jiande granodiorite porphyry shows arc magma geochemical features (e.g., Nb, Ta and Ti depletion), with bulk Earth‐like εNd (t) values (?2.89 to ?1.92), εHf (t) values (?0.6 to +2.8), and initial 87Sr/86Sr (0.7078 to 0.7105). However, a non‐arc setting in the Middle Jurassic is indicated by the absence of arc rocks and the presence of rifting‐related igneous rock associations in the interior of South China. Combined with the regional Neoproterozoic Jiangnan Orogeny, it indicates that these arc magma geochemical features are possibly inherited from the Neoproterozoic juvenile continental crust formed by the ancient oceanic crust subduction along the Jiangnan Orogen. The geodynamic environment that is responsible for the development of the Middle Jurassic Jiande granodiorite porphyry is likely a localized intra‐continental extensional environment along the NE‐trending Jiangshan‐Shaoxing Deep Fault as a tectonic response to far‐field stress at the margins of the rigid South China Plate during the early stage of the paleo‐Pacific plate subduction. In terms of Cu mineralization, we suggest that the metal Cu was released from the subducted oceanic slab and reserved in the juvenile crust during Neoproterozoic subduction along the eastern Jiangnan Orogen region. Partial melting of the Cu rich Neoproterozoic juvenile crust during the Middle Jurassic time in the Jiande area caused the formation of adakitic rocks and the Cu deposit.  相似文献   

18.
西藏努日晚白垩世埃达克岩:洋脊俯冲的产物   总被引：2，自引：0，他引：2  
代作文  李光明  丁俊  黄勇  曹华文 《地球科学》2018,43(8):2727-2741
前人对冈底斯带晚白垩世埃达克岩的成因和地球动力学背景存在不同的认识.对努日地区的石英闪长玢岩开展了锆石U-Pb年代学、全岩地球化学及Hf同位素研究.结果表明,努日石英闪长玢岩侵位于96.5±1.3 Ma,以高SiO2（63.96%~65.75%）、Al2O3（14.37%~15.99%）、MgO（2.12%~2.39%）、Sr（362×10-6~575×10-6,平均为467×10-6）含量,低Y（8.94×10-6~11.50×10-6）、Yb（0.81×10-6~1.06×10-6）含量及高Sr/Y比值（33.52~60.65）为特征,显示埃达克岩地球化学特征.岩石属低钾-中钾钙碱性准铝质花岗岩（A/CNK=0.81~0.96）,富集LREE、亏损HREE,富集大离子亲石元素（LILE）、亏损高场强元素（HFSE）,无明显负Eu异常.锆石εHf（t）值为-0.3~+15.2（主要为+10.0~+15.2）,二阶段模式年龄tDM2为187~1 173（主要为187~516 Ma）,表明源区以俯冲洋壳为主,并可能卷入了少量俯冲沉积物.岩石具有较高的Mg#值和相容元素Cr、Ni含量,表明熔体在上升过程中与地幔发生了相互作用.通过与南冈底斯典型埃达克岩对比,认为努日石英闪长玢岩是在洋脊俯冲背景下、穿过板片窗的高热流导致板片窗边缘的洋壳（及少量俯冲沉积物）部分熔融形成的.   相似文献   

19.
俯冲带部分熔融   总被引：3，自引：3，他引：0  
张泽明  丁慧霞  董昕  田作林 《岩石学报》2020,36(9):2589-2615
俯冲带是地幔对流环的下沉翼，是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度，因此，一般认为俯冲板片并不会发生部分熔融，而是脱水导致上覆地幔楔发生部分熔融。但是，也有研究认为，在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下，俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时，洋壳物质可以发生饱和水或脱水熔融，基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构，俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比，俯冲的大陆地壳具有很低的水含量，较难发生部分熔融，但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融，形成S型花岗岩和埃达克质岩石。  相似文献   

20.
Melting of subducting oceanic lithosphere and associated melt-mantle interactions in convergent plate margins require specific geodynamic environment that allows the oceanic slab to be abnormally heated. Here we focus on the Early Mesozoic mafic rocks and granite porphyry, which provide insights into slab melting processes associated with final closure of the Paleo-Asian Ocean. The granite porphyry samples are calc-alkaline and distinguished by high Sr contents, strong depletion of heavy rare earth elements, resulting in high (La/Yb)N and Sr/Y ratios, and negligible Eu anomalies. Based on their high Na2O and MgO, low K2O contents, positive εHf(t) and εNd(t) and low (87Sr/86Sr)i values, we propose that the granite porphyry was likely derived from partial melting of subducting Paleo-Asian oceanic crust. The Nb-enriched mafic rocks are enriched in Rb, Th, U, Pb and K, and depleted in Nb, Ta, Ba, P and Ti, corroborating a subduction-related origin. Their heterogeneous Sr-Nd-Hf-O isotopic compositions and other geochemical features suggest that they were likely derived from partial melting of peridotitic mantle wedge interacted with oceanic slab-derived adakitic melts. Trace element and isotope modeling results and elevated zircon δ18O values suggest variable subducting sediments input into the mantle wedge, dominated by terrigenous sediments. Synthesizing the widely-developed bimodal rock associations, conjugated dikes, thermal metamorphism, tectonic characteristics, paleomagnetic constraints, and paleogeographical evidence along the Solonke-Changchun suture zone, we identify a slab window triggered by slab break-off, which accounts for slab melting and formation of the Nb-enriched mafic rocks and associated adakitic granite porphyry in southeastern Central Asian Orogenic Belt.  相似文献   

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