Controls on low-pressure anatexis   总被引：2，自引：0，他引：2  

C. C. GERBI  S. E. JOHNSON  P. O. KOONS 《Journal of Metamorphic Geology》2006,24(2):107-118

Low-pressure anatexis, whereby rocks melt in place after passing through the andalusite stability field, develops under more restricted conditions than does low-pressure metamorphism. Our thermal modelling and review of published work indicate that the following mechanisms, operating alone, may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust: (i) magmatic advection by pervasive flow; (ii) crustal-scale detachment faulting; and (iii) the presence of a high heat-producing layer. Of these, only magmatic advection by pervasive flow and crustal-scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting. Combinations of the above mechanisms with pluton-scale magmatic advection, shear heating, removal of the lithospheric mantle, or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low-pressure anatexis.  相似文献   

河北平山湾子群夕线石英集合体的地质地球化学特征及成因   总被引：1，自引：0，他引：1       下载免费PDF全文

万渝生  黄增芳杨崇辉刘增效 《中国地质》2003,30(2):151-158

河北平山地区早前寒武纪湾子群及相关岩石中夕线石英集合体成因存在长期争论。根据地质地球化学研究，本文将它们划分为3种不同类型。(1)球状和团块状夕线石英集合体，其形成与钾长浅粒岩的深熔作用有关。(2)面状夕线石英集合体，与钾长浅粒岩在应力状态下的变质分异作用有关。两者地球化学组成及与相邻浅色体关系受母岩和作用过程双重因素制约。(3)层状夕线石英集合体，为变质沉积成因，地球化学上反映了变质原岩的组成特征。  相似文献   

A comparison of the evolution of diamondiferous quartz-rich rocks from the Saxonian Erzgebirge and the Kokchetav Massif: are so-called diamondiferous gneisses magmatic rocks?     

Hans-Joachim Massonne 《Earth and Planetary Science Letters》2003,216(3):347-364

The petrography and chemical composition of minerals of quartz-rich diamondiferous rocks from the Kokchetav Massif, especially the zonation of garnet, were studied and compared with diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge. Many compositional and textural features were found to be similar. For instance, microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks. On the basis of experimental data, a magmatic scenario was constructed to check if the quartz-rich diamondiferous rocks are of magmatic origin. By this, the P-T paths, derived here for the Kokchetav rocks, and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts. These melts originated by anatexis of deeply submerged metasediments (Erzgebirge: at T as high as 1200°C, Kokchetav Massif: at 50-100°C lower T) and ascended from at least 200 km depth. Relics of the pre-anatectic evolution are still present, for instance, as garnet cores. After ascent and emplacement of the magma in deep portions of thickened continental crust (Kokchetav Massif: 45-50 km close to 800°C, Erzgebirge: 55-60 km at 30-50°C lower T) considerable quantities of (white and/or dark) micas formed by peritectic reactions from melt. For instance, garnets could be resorbed at this stage and biotite grew instead. After the magmatic stage, retrogression took place much stronger in the Kokchetav Massif. This was accompanied by deformation transforming broadly the magmatic texture of quartz-rich diamondiferous rocks from the Kokchetav Massif to a gneissic texture.  相似文献   

吕梁山北段变质岩系变形分解作用初探   总被引：2，自引：0，他引：2  

米广尧  苗培森 《华北地质矿产杂志》1997,12(2):167-175

吕梁山北段变岩系中，变形分解现象从宏观到微观的各级尺度上均有表现。平面上强弱变形带（域）常呈网结状形态产出，剖面上随深度的加深，强弱变形带（域）互为消长。在变形分解作用一，角闪石转化为黑云母、绿泥石、斜长石牌号降低。固流限较低的矿物发生溶解，或充填缍裂隙形成同构造分泌结晶脉，或在一定条件下形成变斑晶，致使岩石的结构构造由块状向片麻状、片状转化。Ｍｇ、Ｆｅ、Ｔｉ、Ｐ、Ｋ、Ｒｂ增高，Ｓｉ、Ｎａ、Ｓｒ降  相似文献   

Silica-rich Melts in Quartz Xenoliths from Vulcano Island and their Bearing on Processes of Crustal Anatexis and Crust-Magma Interaction beneath the Aeolian Arc, Southern Italy     

FREZZOTTI  MARIA-LUCE; PECCERILLO  ANGELO; ZANON  VITTORIO; NIKOGOSIAN  IGOR 《Journal of Petrology》2004,45(1):3-26

Quartz-rich xenoliths in lavas and pyroclastic rocks from VulcanoIsland, part of the Aeolian arc, Italy, contain silicic meltinclusions with high SiO₂ (73–80 wt %) and K₂O (3–6wt %) contents. Two types of inclusions can be distinguishedbased on their time of entrapment and incompatible trace element(ITE) concentrations. One type (late, ITE-enriched inclusions)has trace element characteristics that resemble those of themetamorphic rocks of the Calabro-Peloritano basement of theadjacent mainland. Other inclusions (early, ITE-depleted) havevariable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element(REE) contents. Their REE patterns are unfractionated, witha marked positive Eu anomaly. Geochemical modelling suggeststhat the ITE-depleted inclusions cannot be derived from equilibriummelting of Calabro-Peloritano metamorphic rocks. ITE-enrichedinclusions can be modelled by large degrees (>80%) of meltingof basement gneisses and schists, leaving a quartz-rich residuerepresented by the quartz-rich xenoliths. Glass inclusions inquartz-rich xenoliths represent potential contaminants of Aeolianarc magmas. Interaction between calc-alkaline magmas and crustalanatectic melts with a composition similar to the analysed inclusionsmay generate significant enrichment in potassium in the magmas.However, ITE contents of the melt inclusions are comparablewith or lower than those of Vulcano calc-alkaline and potassicrocks. This precludes the possibility that potassic magmas inthe Aeolian arc may originate from calc-alkaline parents throughdifferent degrees of incorporation of crustal melts. KEY WORDS: melt inclusions; crustal anatexis; magma assimilation; xenoliths; Vulcano Island  相似文献   

Using monazite and zircon petrochronology to constrain the P–T–t evolution of the middle crust in the Bhutan Himalaya          下载免费PDF全文

D. Regis  C. J. Warren  C. M. Mottram  N. M. W. Roberts 《Journal of Metamorphic Geology》2016,34(6):617-639

The growth and dissolution behaviour of accessory phases (and especially those of geochronological interest) in metamorphosed pelites depends on, among others, the bulk composition, the prograde metamorphic evolution and the cooling path. Monazite and zircon are arguably the most commonly used geochronometers for dating felsic metamorphic rocks, yet crystal growth mechanisms as a function of rock composition, pressure and temperature are still incompletely understood. Ages of different growth zones in zircon and monazite in a garnet‐bearing anatectic metapelite from the Greater Himalayan Sequence in NW Bhutan were investigated via a combination of thermodynamic modelling, microtextural data and interpretation of trace‐element chemical ‘fingerprint’ indicators in order to link them to the metamorphic stage at which they crystallized. Differences in the trace‐element composition (HREE, Y, Eu_N/Eu*_N) of different phases were used to track the growth/dissolution of major (e.g. plagioclase, garnet) and accessory phases (e.g. monazite, zircon, xenotime, allanite). Taken together, these data constrain multiple pressure–temperature–time (P–T–t) points from low temperature (<550 °C) to upper amphibolite facies (partial melting, >700 °C) conditions. The results suggest that the metapelite experienced a cryptic early metamorphic stage at c. 38 Ma at <550 °C, ≥0.85 GPa during which plagioclase was probably absent. This was followed by a prolonged high‐T, medium‐pressure (~600 °C, 0.55 GPa) evolution at 35–29 Ma during which the garnet grew, and subsequent partial melting at >690 °C and >18 Ma. Our data confirm that both geochronometers can crystallize independently at different times along the same P–T path and that neither monazite nor zircon necessarily provides timing constraints on ‘peak’ metamorphism. Therefore, collecting monazite and zircon ages as well as major and trace‐element data from major and accessory phases in the same sample is essential for reconstructing the most coherent metamorphic P–T–t evolution and thus for robustly constraining the rates and timescales of metamorphic cycles.  相似文献   

喜马拉雅碰撞造山带不同类型部分熔融作用的时限及其构造动力学意义   总被引：2，自引：0，他引：2  

高利娥  曾令森  王莉  侯可军  高家昊  尚振 《地质学报》2016,90(11):3039-3059

喜马拉雅新生代淡色花岗岩,是世界上S型花岗岩的典例,主要分布于两条近平行排列的东西向构造带内,特提斯喜马拉雅带和高喜马拉雅带。实验岩石学和理论研究表明:这些淡色花岗岩是中—下地壳岩石进行不同性质的地壳深熔作用的产物,部分熔融类型与构造变形密切耦合。具体表现在:146~35 Ma,在增厚地壳条件下,以角闪岩部分熔融作用为主,形成了具有高Sr/Y比值的二云母花岗岩;228~9 Ma,减压条件下,俯冲物质快速折返,白云母发生脱水部分熔融,形成具有较高Rb/Sr比值的花岗岩;3其中,在21~16 Ma期间,与藏南裂谷系E—W向伸展作用开启密切相关,变泥质岩发生水致白云母部分熔融作用,形成Rb/Sr比值较低,Sr和Ba含量较高的花岗岩;4在25~27 Ma期间,局部地区发生高压水致部分熔融作用。  相似文献   

西藏冈底斯带阿索构造混杂岩南侧亚布努马花岗斑岩地球化学特征及锆石U-Pb年龄     

李航  王明  解超明  曾孝文  董宇超  于云鹏  罗安波 《地质通报》2018,37(8):1510-1518

西藏中冈底斯北部尼玛县阿索乡亚布努马地区东侧出露一处花岗斑岩岩脉,LA-ICP-MS锆石U-Pb测年结果显示,该花岗斑岩的形成时代为晚侏罗世(161.2±5.9Ma)。全岩地球化学数据显示其高硅、富碱、富铝的特征,属于碱性准铝质花岗斑岩;富集轻稀土元素,轻、重稀土元素分异明显,具有明显的负Eu异常,富集Rb、Pb等大离子亲石元素,亏损Ba、Sr元素及Nb、Ta、Ti、U等高场强元素,形成于岛弧环境。其源区可能为来自俯冲带增厚下地壳的深熔作用,结合区域上同时代的岩浆事件,亚布努马花岗斑岩应该形成于以班公湖-怒江洋南向俯冲为动力背景的陆缘弧环境。  相似文献   

The cordierite‐bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low‐pressure anatexis,melt productivity,melt loss and the preservation of cordierite     

C. GROPPO  F. ROLFO  P. MOSCA 《Journal of Metamorphic Geology》2013,31(2):187-204

Cordierite‐bearing anatectic rocks inform our understanding of low‐pressure anatectic processes in the continental crust. This article focuses on cordierite‐bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite‐bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two‐mica metapelite/metagreywacke to biotite‐metapelite) has been investigated at low‐pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low‐pressure conditions, the two‐mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P‐T paths with a peculiar dP/dT slope of about 15–18 bar °C^?1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low‐P regional anatectic terrane. The cordierite‐bearing migmatites may well represent the source rocks for the Miocene andalusite‐bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low‐P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.  相似文献   

Infiltration-driven dehydration and anatexis in granulite facies metagabbro, Grenville Province, Ontario, Canada   总被引：2，自引：0，他引：2  

D. R. M. PATTISON 《Journal of Metamorphic Geology》1991,9(3):315-332

Dark hornblende + garnet-rich, quartz-absent metagabbro boudins from the Seguin subdomain, Ontario Grenville Province, are transected by anastomosing light-coloured veins rich in orthopyroxene, clinopyroxene, plagioclase and sometimes quartz. The veins vary in texture from fine-grained diffuse veins and patches that overprint the metagabbro, to coarse tonalitic leucosomes with sharp borders. The diffuse veins and patches are suggestive of channellized subsolidus dehydration of the metagabbro, while the tonalitic leucosomes are suggestive of local internally-derived anatexis. All vein types grade smoothly into each other, with the tonalitic leucosomes being the latest.
Relative to the host metagabbro, the veins have higher Si, Na, Ba & Sr, lower Fe, Mg, Ca & Ti, and similar Al. The coarser veins are enriched in K. Plagioclase becomes steadily enriched in Na in the transition from host metagabbro (An₄₇) to the veins (An₃₅), and in the coarsest veins it is antiperthitic. Differences in composition of the other minerals between host metagabbro and vein are minor. Pressure–temperature estimates are scattered, but indicate a minimum temperature during vein formation of 700°C at about 8 kbar.
Mass balance constraints indicate that the veins formed from the metagabbro in an open system. The transecting veins are interpreted to represent pathways of Si + Na + Ba + Sr ± K ± Al-enriched, low a _H2O fluids that metasomatized the host metagabbro to form the anhydrous veins. An initial period of localized solid-state dehydration of the metagabbro, represented by the diffuse veins, was followed by a transition to localized anatexis, represented by the tonalitic leucosomes. The change to anatexis may have been due to the addition of K to the infiltrating fluid. The source and delivery mechanism of the fluids is unknown.  相似文献