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1.
TTGs and adakites: are they both slab melts? 总被引：21，自引：0，他引：21
Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings.
Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 °C.
Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni. These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth. 相似文献
2.
俯冲带部分熔融 总被引：3，自引：3，他引：0
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
3.
中国东部富钾埃达克岩成因的实验约束 总被引：27，自引：21，他引：27
Adakite在地球化学上具明显特征的火山岩和深成花岗岩类岩石，见于洋内岛孤环境和大陆孤，如安底斯孤。在洋内岛孤，由热的消减的大洋岩石圈熔融形成（叫做“板片熔融”），而在大陆孤，熔融曾发生在构造或岩浆加厚的下地壳底（叫做“下地壳熔融”）。在这两种产状环境中，adakite的鲜明地球化学特征被认为是起因子，一种不同程度含水的变质基性原岩在足够深度上的部分熔融，这里的足够深度是指可使石榴子石在残余结晶组合（即石榴角闪石和/或榴辉石的残余）中保持稳定的深度。“原始”或“母”adakite熔体一旦形成，便可能在其向上运移和侵位上地壳期间受到同化作用（或是地幔，或是大陆物质）和结晶分异作用的改造。中国东部晚中生代（早中白垩世，160-110Ma）的adakite,与见于同一地区和其它地方的钠质adakite相比，通常富含钾(K2O)和其它大离子亲石元素（如Ba,Th,U），有较低的Na2O/K2O比值（-1.0-1.1）,类似于玄武岩在石榴角闪岩-榴辉岩相含水熔融实验中所产生adakite熔体，要么是由洋壳板片熔融所形成，要么是由不同成分的玄武质下地壳原岩部分熔融所形成。尽管有些成分差异，它们的总体化学特征仍然可将中国东部的富钾花岗岩类岩石定均adakite。我们把这些富钾的adakite的独特化学行特征，归因于成分来源的特殊性，或adakite母岩浆遭受了同化混染和结晶分异(AFC)作用的改造。虽然中国东部与消减带环境明显不同这一点表明，那里的adakite可由板块底部侵位的（岩浆加厚的）镁铁质下地壳部分熔融所形成，但燕山运动期间中国东部存在“平坦”俯冲的地球动力学环境是可能被排除的。 相似文献
4.
Herv Martin 《Lithos》1993,30(3-4):373-388
The petrographic and chemical composition of magmatic rocks generated during the Archaean appears to be different from that of post-Archaean rocks. Komatiites are widespread before 2.5 Ga and rarely occur afterwards. In addition the Archaean continental crust is primarily TTG (Tonalitic, Trondhjemitic and Granodioritic) in composition, exhibiting typical trondhjemitic differentiation trends; whereas modern equivalents are granodioritic to granitic following classical calc-alkaline differentiation trends. This distinction becomes more prominent when rare-earth elements (REE) are taken into account: Archaean TTG are Yb-poor (YbN < 8.5) and have high (La/Yb) ratios (5 < (La/Yb)N < 150), in comparison, the post-2.5 Ga granitoids, emplaced in subduction-zone geodynamic environments have high Yb content (4.5N<20) with very low (La/Yb)N ratios ( 20). Theoretical calculations and experimental petrology have shown that the TTG can be produced by partial melting of an Archaean tholeiite transformed into garnet-bearing amphibolite. Consequently, the low heavy REE content of the TTG is explained by the influence of both residual garnet and hornblende in their source. After 2.5 Ga the role of these minerals in calc-alkaline magma genesis becomes progressively less important, which is interpreted in terms of a cooling Earth model.
In modern subduction zone environments the subducted oceanic slab is relatively “old and cold” and the geothermal gradient along the Benioff plane in low (ca. 10°C/km). Consequently, the down-going lithosphere undergoes dehydration before partial melting is able to occur. The liberated fluids are light REE and LILE-enriched and ascend into the overlying mantle wedge where they induced partial fusion. The produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes. Mantle derived magmas typically exhibit high Yb contents due to low KDYb values for olivine and pyroxenes. During the Archaean, the subducted lithosphere was relatively “young and hot” providing high geothermal gradients along the Benioff zone. Thus, partial melting of the subducted slab was possible at lower temperatures before dehydration would take place. Garnet and hornblende are the main residual phases accounting for the low Yb contents of the Archaean TTG.
This model can be tested using a modern analogue of Archaean-like subduction processes. In south Chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the Benioff zone. Thus in very small areas, Archaean-like environments may be simulated in modern subduction zones. The modern andesites produced in this environment show Archaean geochemical characteristics with low YbN (<5), whereas the majority of andesites along the Andean arc have modern patterns with YbN ranging from 8 to more than 17. This conclusion was generalised to all young subducted lithospheres all over the world.
In conclusion, it appears that since the Archaean there has been a change in the site of continental crust genesis. The location of calc-alkaline magma source in subduction-zone environments has migrated through time from the subducted slab to the mantle wedge. This is a direct consequence of the progressive cooling of the Earth. 相似文献
5.
Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands, south of mainland New Zealand, are porphyritic trachyandesites and andesites (58.20–62.19 wt% SiO2) with phenocrysts of amphibole, plagioclase and biotite. The Solander and Little Solander rocks are incompatible element enriched (e.g. Sr ~931–2,270 ppm, Ba ~619–798 ppm, Th ~8.7–21.4 ppm and La ~24.3–97.2 ppm) with MORB-like Sr and Nd isotopic signatures. Isotopically similar quench-textured enclaves reflect mixing with intermediate (basaltic-andesite) magmas. The Solander rocks have geochemical affinities with adakites (e.g. high Sr/Y and low Y), whose origin is often attributed to partial melting of subducted oceanic crust. Solander sits on isotopically distinct continental crust, thus excluding partial melting of the lower crust in the genesis of the magmas. Furthermore, the incompatible element enrichments of the Solander rocks are inconsistent with partial melting of newly underplated mafic lower crust; reproduction of their major element compositions would require unrealistically high degrees of partial melting. A similar argument precludes partial melting of the subducting oceanic crust and the inability to match the observed trace element patterns in the presence of residual garnet or plagioclase. Alternatively, an enriched end member of depleted MORB mantle source is inferred from Sr, Nd and Pb isotopic compositions, trace element enrichments and εHf ? 0 CHUR in detrital zircons, sourced from the volcanics. 10Be and Sr, Nd and Pb isotopic systematics are inconsistent with significant sediment involvement in the source region. The trace element enrichments and MORB-like Sr and Nd isotopic characteristics of the Solander rocks require a strong fractionation mechanism to impart the high incompatible element concentrations and subduction-related (e.g. high LILE/HFSE) geochemical signatures of the Solander magmas. Trace element modelling shows that this can be achieved by very low degrees of melting of a peridotitic source enriched by the addition of a slab-derived melt. Subsequent open-system fractionation, involving a key role for mafic magma recharge, resulted in the evolved andesitic adakites. 相似文献
6.
7.
Magma genesis beneath Northeast Japan arc: A new perspective on subduction zone magmatism 总被引：3，自引：2，他引：1
It is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrous minerals. In the context of this “dehydration embrittlement” hypothesis, we propose a new model to explain key features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basaltic systems determined by thermodynamic calculations and seismic structures of Northeast Japan arc revealed by latest seismic studies. The model predicts that partial melting of basaltic crust in the subducting slab is an inevitable consequence of subduction of hydrated oceanic lithosphere. Aqueous fluids released from the subducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to just below overlying crust at volcanic front. Hydrous minerals in the mantle wedge are stable only in shallow (< 120 km) areas, and are absent in the layer that is dragged into deep mantle by the subducting slab. The position of volcanic front is not restricted by dehydration reactions in the subducting slab but is controlled by dynamics of mantle wedge flow, which governs the thermal structure and partial melting regime in the mantle wedge. 相似文献
8.
Dehydration melting of a basaltic composition amphibolite at 1.5 and 2.0 GPa: implications for the origin of adakites 总被引：50，自引：0，他引：50
This study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm (1.5 and 2.0 GPa and temperatures of 850 to 1150° C). Dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts. At 1.5 GPa, the amphibolite melts in two stages between 800 and 1025° C. The 2.0 GPa data also define two melting stages. At 2.0 GPa, the first stage involves nearly modal melting of the original amphibolite minerals (qtz, pl, amp) to produce melt + cpx + grt. During the second stage, the eclogite restite melts non-modally (0.86 cpx + 0.14 grt = 1 melt). The experimental results were combined with data from the literature to generate a composite P-T phase diagram for basaltic composition amphibolites over the 800 to 1100° C temperature range for pressures up to 2.0 GPa. Comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts (adakites) shows that partial melting of amphibolite at conditions appropriate to a hot-slab geotherm produces melts similar to andesitic and dacitic adakites except for significant MgO and CaO depletions. Trace element modelling of amphibolite dehydration melting using the 2.0 GPa melting reactions produces REE abundances similar to those of adakites at 10–15 wt% batch melting, but the models do not reproduce the high Sr/Y ratios characteristic of adakites. Taken together, the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust. If adakites are partial melts of the subducted slab, they must undergo significant interaction with the mantle and/or crust, during which they acquire a number of their distinctive characteristics. 相似文献
9.
Hf-Nd isotope evidence for contemporaneous subduction processes in the source of late Archean arc lavas from the Superior Province, Canada 总被引：2，自引：0，他引：2
Volcanic suites from Wawa greenstone belts in the southern Superior Province comprise an association of typical late Archean arc volcanic rocks including adakites, magnesian andesites (MA), niobium-enriched basalts (NEB), and ‘normal’ tholeiitic to calc-alkaline basalts to rhyolites. The adakites represent melts from subducted oceanic crust and all other suites were derived from the mantle wedge above the subducting oceanic lithosphere. The magnesian andesites are interpreted to be the product of hybridization of adakite melts with arc mantle wedge peridotite. The initial ?Hf values of the ∼2.7 Ga Wawa adakites (+3.5 to +5.2), magnesian andesites (+2.6 to +5.1), niobium-enriched basalts (+4.4 to +6.6), and ‘normal’ tholeiitic to calc-alkaline arc basalts (+5.3 to +6.4) are consistent with long-term depleted mantle sources. The niobium-enriched basalts and ‘normal’ arc basalts have more depleted ?Hf values than the adakites and magnesian andesites. The initial ?Nd values in the magnesian andesites (+0.4 to +2.0), niobium-enriched basalts (+1.4 to +2.4), and ‘normal’ arc tholeiitic to calc-alkaline basalts (+1.6 to +2.9) overlap with, but extend to lower values than, the slab-derived adakites (+2.3 to +2.8). The lower initial ?Nd values in the mantle-wedge-derived suites, particularly in the magnesian andesites, are attributed to recycling of an Nd-enriched component with lower ?Nd to the mantle wedge. As a group, the slab-derived adakites plot closest to the 2.7 Ga depleted mantle value in ?Nd versus ?Hf space, additionally suggesting that the Nd-enriched component in the mantle wedge did not originate from the 2.7 Ga slab-derived melts. Accordingly, we suggest that the enriched component had been added to the mantle wedge at variable proportions by recycling of older continental material. This recycling process may have occurred as early as 50-70 Ma before the initiation of the 2.7 Ga subduction zone. The selective enrichment of Nd in the sources of the Superior Province magmas can be explained by experimental studies and geochemical observations in modern subduction systems, indicating that light rare earth elements (e.g., La, Ce, Sm, Nd) are more soluble than high field strength elements (e.g., Zr, Hf, Nb, Ta) in aqueous fluids that are derived from subducted slabs. As a corollary, we suggest that the recycled Nd-enriched component was added to the mantle source of the Wawa arc magmas by dehydration of subducted sediments. 相似文献
10.
Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines) 总被引：28，自引：0，他引：28
Fernando G. Sajona Ren C. Maury Manuel Pubellier Jacques Leterrier Herv Bellon Joseph Cotten 《Lithos》2000,54(3-4):173-206
Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na2O/K2O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere. 相似文献
11.
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
12.
For the last two decades, Iceland and other oceanic plateaux have been considered as potential analogues for the formation of the early Earth's continental crust. This study examines the compositions of silicic rocks from modern oceanic plateaux, revealing their differences to Archaean continental rock types (trondhjemite–tonalite–granodiorite or TTG) and thereby emphasising the contrasted mechanisms and/or sources for their respective origins. In most oceanic plateaux, felsic magmas are thought to be formed by fractional crystallization of basalts. In Iceland, the interaction between mantle plume and the Mid‐Atlantic ridge results in an abnormally high geothermal gradient and melting of the hydrated metabasaltic crust. However, despite the current `Archaean‐like' high geothermal gradients, melting takes place at a shallow depth and is unable to reproduce the TTG trace element signature. Consequently, oceanic plateaux are not suitable environments for the genesis of the Archaean continental crust. However, their subduction could account for the episodic crustal growth which has occurred throughout the Earth's history. 相似文献
13.
前人对深俯冲板片释放熔/流体交代地幔楔形成弧岩浆源区的过程和机制已得到充分认识,然而对地幔楔岩石能否脱水交代深俯冲地壳并不清楚.在对欧洲西阿尔卑斯造山带Dora-Maira地体白片岩的地球化学研究中,首次发现地幔楔交代岩能够脱水反向交代深俯冲地壳岩石,从而极大影响俯冲地壳的地球化学组成.结合白片岩和围岩的全岩地球化学特征以及锆石学结果,查明了白片岩的原岩为S型花岗岩,澄清了关于Dora-Maira白片岩原岩属性的长期争议.在此基础上,发现白片岩中变质锆石相对残留岩浆锆石δ18O值显著降低,指示原岩形成后受到低δ18O变质流体的交代作用.白片岩具有高温岩石中最高的δ26Mg值达0.75‰,显著高于围岩变花岗岩,指示交代流体具有重Mg同位素组成.基于地球主要岩石储库的Mg同位素组成,推测交代流体来自俯冲隧道中富滑石地幔楔蛇纹岩在弧下深度的脱水分解,而这些地幔楔蛇纹岩是新特提斯洋壳在弧前深度变质脱水产生的流体与地幔楔浅部橄榄岩反应形成.这些结果不仅提供了利用Mg-O同位素示踪俯冲隧道中流体来源的新思路,也提供了地幔楔蛇纹岩来源流体反向交代深俯冲地壳岩石的首个典型实例.这种反向交代不仅极大改变了深俯冲地壳的地球化学组成,而且对弧岩浆岩重Mg同位素成因具有重要意义. 相似文献
14.
Svend Duggen Maxim Portnyagin Joel Baker Kaj Hoernle Nathalie Grassineau 《Geochimica et cosmochimica acta》2007,71(2):452-480
The shift of lava geochemistry between volcanic front to rear-arc volcanoes in active subduction zones is a widespread phenomenon. It is somehow linked to an increase of the slab surface depth of the subducting oceanic lithosphere and increasing thickness of the mantle wedge and new constraints for its causes may improve our understanding of magma generation and element recycling in subduction zones in general. As a case study, this paper focuses on the geochemical composition of lavas from two adjacent volcanic centres from the volcanic front (VF) to rear-arc (RA) transition of the Southern Kamchatkan subduction zone, with the aim to examine whether the shift in lava geochemistry is associated with processes in the mantle wedge or in the subducted oceanic lithosphere or both. The trace element and O-Sr-Nd-Hf-Pb (double-spike)-isotopic composition of the mafic Mutnovsky (VF) and Gorely (RA) lavas in conjunction with geochemical modelling provides constraints for the degree of partial melting in the mantle wedge and the nature of their slab components. Degrees of partial melting are inferred to be significantly higher beneath Mutnovsky (∼18%) than Gorely (∼10%). The Mutnovsky (VF) slab component is dominated by hydrous fluids, derived from subducted sediments and altered oceanic crust, eventually containing minor but variable amounts of sediment melts. The composition of the Gorely slab component strongly points to a hydrous silicate melt, most likely mainly stemming from subducted sediments, although additional fluid-contribution from the underlying altered oceanic crust (AOC) is likely. Moreover, the Hf-Nd-isotope data combined with geochemical modelling suggest progressive break-down of accessory zircon in the melting metasediments. Therefore, the drastic VF to RA shift in basalt chemistry mainly arises from the transition of the nature of the slab component (from hydrous fluid to melt) in conjunction with decreasing degrees of partial melting within ∼15 km across-arc. Finally, systematic variations of key inter-element with high-precision Pb-isotope ratios provide geochemical evidence for a pollution of the Mutnovsky mantle source with Gorely melt components but not vice versa, most likely resulting from trench-ward mantle wedge corner flow. We also present a geodynamic model integrating the location of the Mutnovsky and Gorely volcanic centres and their lava geochemistry with the recently proposed thermal structure of the southern Kamchatkan arc and constraints about phase equilibria in subducted sediments and AOC. Herein, the slab surface hosting the subducted sediments suffers a transition from dehydration to melting above a continuously dehydrating layer of AOC. Wider implications of this study are that an onset of (flush-) sediment melting may ultimately be the main trigger for the VF to RA transition of lava geochemistry in subduction zones. 相似文献
15.
新疆北部晚古生代埃达克岩、富铌玄武岩组合：古亚洲洋板块南向俯冲的证据 总被引：50，自引：0，他引：50
地球化学研究结果表明，新疆北部富蕴县境内的晚古生代下泥盆统托让格库都克组的安山质岩石具有与埃达克岩非常相似的地球化学特征，它们具有较高的Al2O3，Na2O和Sr含量以及Sr/Y比值；明显亏损重稀土和Y，它们的MORB标准化微量元素蛛网图表现为明显的Nb，Ta负异常和Sr正异常，同时强烈亏损高场强元素。与埃达克岩共生的玄武岩的地球化学特征则与富铌玄武岩一致，表现为Si过饱和及富Na的特征，同时具有较高的Nb，TiO2和P2O5含量，并富集高场强元素。由于埃达克岩和富铌玄武岩的形成均与板块俯冲有关，因此，它们的存在表明，古亚洲洋在早-中泥盆世向南(哈萨克斯坦-准噶尔板块)发生了一次洋壳俯冲作用。 相似文献
16.
冈底斯岩浆弧的形成与演化 总被引：10，自引：6，他引：4
位于青藏高原南部的冈底斯岩浆弧是新特提斯大洋岩石圈长期俯冲导致的中生代岩浆作用的产物,而且在印度与亚洲大陆碰撞过程中叠加了强烈的新生代岩浆作用,是世界上典型的复合型大陆岩浆弧,也是研究增生与碰撞造山作用和大陆地壳生长与再造的天然实验室。基于岩浆、变质和成矿作用研究成果,我们将冈底斯弧的形成与演化历史划分5期,即新特提斯洋早期俯冲、新特提斯洋中脊俯冲、新特提斯洋晚期俯冲、印度-亚洲大陆碰撞和后碰撞期。第1期发生在晚白垩世之前,是以新特提斯洋岩石圈的长期俯冲、地幔楔部分熔融形成钙碱性弧岩浆岩为特征。长期的幔源岩浆作用导致了整个冈底斯弧发生显著的新生地壳生长,并在岩浆弧西部形成了一个大型的与俯冲相关的斑岩型铜矿。第2期发生在晚白垩世,活动的新特提斯洋中脊发生俯冲,软流软圈沿板片窗上涌,使上升的软流圈、地幔楔和俯冲洋壳发生部分熔融,导致了强烈的幔源岩浆作用和显著的新生地壳生长与加厚,并以不同类型和不同成分岩浆岩的同时发育和伴随的高温变质作用为特征。第3期发生在晚白垩世晚期,为新特提斯洋脊俯冲后残余大洋岩石圈的俯冲期,以正常的弧型岩浆作用为特征。第4期发生在古新世至中始新世,伴随印度与亚洲大陆的碰撞,俯冲的新特提斯洋岩石圈回转和断离引起软流圈上涌,诱发了强烈的幔源岩浆作用。在此阶段,大陆碰撞导致的地壳挤压缩短和幔源岩浆的底侵与增生,使冈底斯弧经历了显著的地壳生长和加厚,新生和古老加厚下地壳的高压、高温变质和部分熔融,幔源和壳源岩浆岩的共生和强烈的岩浆混合。所形成的I型花岗岩大多继承了新生地壳弧型岩浆岩的化学成分,并多显出埃达克岩的地球化学特征。在岩浆弧北部形成了一系列与起源于古老地壳花岗岩相关的Pb-Zn矿床。第5期发生在晚渐新世到早-中中新世的后碰撞挤压过程中,以地壳的继续加厚,加厚下地壳的高温变质、部分熔融和埃达克质岩石的形成为特征。在岩浆弧东段南部形成了一系列与起源于新生加厚下地壳埃达克质岩石相关的斑岩型Cu-Au-Mo矿。冈底斯带的多期岩浆、变质与成矿作用为其从新特提斯洋俯冲到印度-亚洲大陆碰撞的构造演化提供了重要限定。 相似文献
17.
Maria I. Marín-Cerón Takuya Moriguti Akio Makishima Eizo Nakamura 《Geochimica et cosmochimica acta》2010,74(3):1104-1121
The contribution of subducted carbonate sediments to the genesis of the Southwestern Colombian arc magmas was investigated using a comprehensive petrography and geochemical analysis, including determination of major and trace element contents and Sr, Nd, Hf and Pb isotope compositions. These data have been used to constrain the depth of decarbonation in the subducted slab, indicating that the decarbonation process continues into the sub-arc region, and ultimately becomes negligible in the rear arc. We propose on the basis of multi-isotope approach and mass balance calculations, that the most important mechanism to induce the slab decarbonation is the infiltration of chemically reactive aqueous fluids from the altered oceanic crust, which decreasingly metasomatize the mantle wedge, triggering the formation of isotopically different primary magmas from the volcanic front (VF) with relatively high 176Hf/177Hf, high 87Sr/86Sr, negative values of εNd and lower Pb isotopes compared to the rear arc (RA).The presence of more aqueous fluids at the volcanic front may increase the degree of decarbonation into carbonate-bearing lithologies. Moreover, with increasing pressure and temperature in the subduction system, the decrease in dehydration of the slab, leads to cessation of fluid-induced decarbonation reactions at the rear arc. This development allows the remaining carbonate materials to be recycled into the deep mantle. 相似文献
18.
Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids 总被引：1，自引：0，他引：1
Anne-Sophie Bouvier Takayuki Ushikubo Noriko T. Kita Aaron J. Cavosie Reinhard Kozdon John W. Valley 《Contributions to Mineralogy and Petrology》2012,163(5):745-768
We report δ7Li, Li abundance ([Li]), and other trace elements measured by ion probe in igneous zircons from TTG (tonalite, trondhjemite,
and granodiorite) and sanukitoid plutons from the Superior Province (Canada) in order to characterize Li in zircons from typical
Archean continental crust. These data are compared with detrital zircons from the Jack Hills (Western Australia) with U–Pb
ages greater than 3.9 Ga for which parent rock type is not known. Most of the TTG and sanukitoid zircon domains preserve typical
igneous REE patterns and CL zoning. [Li] ranges from 0.5 to 79 ppm, typical of [Li] in continental zircons. Atomic ratios
of (Y + REE)/(Li + P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition preserved, supporting the hypothesis that
Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow
rate of Li diffusion. The δ7Li and trace element data constrain the genesis of TTGs and sanukitoids. [Li] in zircons from granitoids is significantly
higher than from zircons in primitive magmas in oceanic crust. TTG zircons have δ7Li (3 ± 8‰) and δ18O in the range of primitive mantle-derived magmas. Sanukitoid zircons have average δ7Li (7 ± 8‰) and δ18O higher than those of TTGs supporting genesis by melting of fluid-metasomatized mantle wedge. The Li systematics in sanukitoid
and TTG zircons indicate that high [Li] in pre-3.9-Ga Jack Hills detrital zircons is a primary igneous composition and suggests
the growth in proto-continental crust in magmas similar to Archean granitoids. 相似文献
19.
富铌玄武岩：板片熔体交代的地幔楔橄榄岩部分熔融产物 总被引：4，自引：0，他引：4
富铌玄武岩是一类具有特殊地球化学特征的岛弧玄武岩。与正常岛弧玄武岩相比，它具有硅饱和并富钠的特征；同时具有相对高的Nb（一般>7×10-6）、TiO2（1%～2%）和P含量，以及低的LILE/HFSE和HREE/HFSE比值，并富集高场强元素；它的原始地幔标准化微量元素图显示了弱的Nb、Ta负异常（有时出现弱的正异常），原始地幔标准化La/Nb比值小于2（但很少小于0.7），它是由受埃达克质熔体交代过的地幔橄榄岩部分熔融形成的。由于富铌玄武岩与埃达克岩是大洋板片俯冲作用的直接产物，因此，通过对该岩石组合及与俯冲作用有关的流体和熔体的研究，不仅可以查明洋壳俯冲作用过程中的岩浆活动特征，还可以阐明洋壳俯冲及壳幔相互作用，具十分重要的地质意义。 相似文献
20.
Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. 40Ar/39Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa. 相似文献