Genesis of the New Quebec and Adirondack granulites: Evidence for their production by partial melting

Materials balance calculations on amphibolite facies and granulite facies rocks from the Adirondack region and from some areas of New Quebec demonstrate that the rocks of the two facies differ by near granite-minimum melt compositions. The implication of these results are; granulites of both regions result from partial melting of greywacke-type metasedimentary rocks during upper amphibolite facies metamorphism. The New Quebec region contains granuites produced by partial melting as well as granulites produced by simple mineral dehydration reactions (no melting of the rocks occurred). These observations combined with the knowledge that some New Quebec granulites have produced abnormally high quantities of melts provide evidence that large sources and sinks for H₂O exist in the lower crust and that there is significant H₂O transport between these regions.     

The petrogenesis of Tertiary cone-sheets in Ardnamurchan, NW Scotland: petrological and geochemical constraints on crustal contamination and partial melting

Abundant cone sheets form one of the last magmatic stages in the Tertiary central complex on the Scottish peninsula of Ardnamurchan and can be grouped into a younger inner and an older outer suite relative to a gabbro intrusion. Most of the cone-sheets consist of tholeiitic to transitional basalt with MgO contents between 7.5% and 4%, although more evolved rocks also occur (to 0.5% MgO). The mafic samples are slightly enriched in the light rare earth elements (Chondrite-normalized La/Sm ∼1.1), the enrichment increases in the more evolved rocks. The compositional variation of the basaltic rocks is mainly due to crystal fractionation of olivine and clinopyroxene at depths of ∼10 km but trace elements show simultaneous assimilation of Archean Lewisian granulite crust. The andesitic to rhyolitic lavas formed by fractional crystallization from the contaminated basaltic magma coupled with assimilation of Proterozoic Moine metasediments at uppermost crustal levels. The occurrence of composite cone-sheets with basaltic and rhyolitic parts and mixtures between these magmas implies that the melts ascended successively but within a short period of time. The parental magmas of the Ardnamurchan cone-sheets must have formed at relatively shallow depths in the mantle and are comparable to the youngest tholeiitic lavas from the neighbouring island of Mull. Received: 5 June 1997 / Accepted: 12 November 1997     

Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study

A combined geochronological, geochemical, and Nd isotopic study of felsic high-pressure granulites from the Snowbird Tectonic Zone, northern Saskatchewan, Canada, has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry (ID-TIMS) techniques. The terrane investigated is a 400 km² domain of garnet–kyanite–K–feldspar-bearing quartzofeldspathic gneisses. Monazite in these granulites preserves a complex growth history from 2.6 to 1.9 Ga, with well-armored, high Y and Th grains included in garnet yielding the oldest U–Pb dates at 2.62 to 2.59 Ga. In contrast, matrix grains and inclusions in garnet rims that are not well-armored are depleted in Y and Th, and display more complicated U–Pb systematics with multiple age domains ranging from 2.5 to 2.0 Ga. 1.9 Ga monazite occurs exclusively as matrix grains. Zircon is typically younger (2.58 to 2.55 Ga) than the oldest monazite. Sm–Nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of Archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial _Nd. In contrast, grains that contain a Paleoproterozoic component show more positive initial _Nd, most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of HREE's. Our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith. The ca. 2.55 Ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed. Very high-pressure (> 1.5 GPa) metamorphism during the Paleoproterozoic at 1.9 Ga produced kyanite from garnet breakdown, and resulted in limited growth of new monazite and zircon. In the case of monazite, this is likely due to the armoring and sequestration of early-formed monazite such that it could not participate in metamorphic reactions during the high-pressure event, as well as the depletion of the REE's due to melt loss following the early melting event.     

Experimental constraints on major and trace element partitioning during partial melting of eclogite

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al₂O₃). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO₂ content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile.Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high-field strength elements indicate complementary D_Zr/D_Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.     

Aluminous granulites from the Pulur complex, NE Turkey: a case of partial melting, efficient melt extraction and crystallisation

In the Pulur complex, NE Turkey, a heterogeneous rock sequence ranging from quartz-rich mesocratic gneisses to silica- and alkali-deficient, Fe-, Mg- and Al-rich melanocratic rocks is characterized by granulite-facies assemblages involving garnet, cordierite, sillimanite, ilmenite, ±spinel, ±plagioclase, ±quartz, ±biotite, ±corundum, rutile and monazite. Textural evidence for partial melting in the aluminous granulites, particularly leucosomes, is largely absent or strongly obliterated by a late-stage hydrothermal overprint. However, inclusion relations, high peak P–T conditions, the refractory modes, bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting. The melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica-rich, bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with SiO₂. Peak conditions are estimated to be ≥800 °C and 0.7–0.8 GPa. Subsequent near-isothermal decompression to 0.4–0.5 GPa at 800–730 °C is suggested by the formation of cordierite coronas and cordierite–spinel symplectites around garnet and in the matrix. Sm–Nd, Rb–Sr and ⁴⁰Ar/³⁹Ar isotope data indicate peak conditions at 330 Ma and cooling below 300 °C at 310 Ma.     

Criteria for the recognition of partial melting

Partial melting changes rocks from single phase (solid) to two phase (solid+melt) systems. The bulk viscosity decreases as the melt fraction increases and this effect raises the rate of deformation and heat transfer, as well as causing crustal differentiation. Therefore, it is important to be able to recognise which rocks have partially melted.Macroscopic textures provide the simplest criteria for recognising partial melting. Melting and deformation are generally synchronous, and when the melt fraction retained is low (<20%) metatexite migmatites are formed. Typically, these are morphologically complex because the melt fraction is squeezed out of the deforming matrix and collects in whatever dilatant sites are present. The presence of melanosome layers and patches provides the best evidence of where the melt formed, and the leucosomes where it collected. Diatexite migmatites can be easily recognised by the presence of a flow foliation, schlieren, enclaves and vein like leucosomes, and are evidence of a high melt fraction and pervasive partial melting. For the unusual case of melting without synchronous deformation, rounded neosome patches containing both the melt and solid fractions of the melt-producing reaction develop and, as the degree of melting increases these enlarge, to form diatexite migmatites. In both cases the characteristic feature is an increased grainsize and loss of pre-migmatization structures. Migmatite textures are robust, they survive later deformation well.Microscopic textures such as: (1) thin films of quartz, plagioclase and K-feldspar along brain boundaries that represent crystallized melt and, (2) melt-solid reaction textures, also provide good criteria for identifying partially melted rocks. However, these textures are fragile and easily destroyed by deformation. The identification of mineral assemblages from which melt-forming reactions can be inferred is another reliable critera for recognising partial melting, but post-migmatization rehydration in granulite terranes can destroy this evidence.Whole rock geochemistry can be used to model the partial melting process, but problems in identifying the palaeosome and an unmodified melt compositions can restrict its application. However, whole rock geochemistry coupled with good field based control, can be used to deduce what processes have occurred in a terrane where the rocks have partially melted.Variations in field appearance, texture and composition are, in large part a consequence of whether, or not, and when, the melt-fraction separated from the solid fraction.     

Impact-induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites

Mid‐crustal Archean pelitic granulites in the Vredefort Dome experienced a static, low‐P granulite facies overprint associated with the formation of the dome by meteorite impact at 2.02 Ga. Heating and exhumation were virtually instantaneous, with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave. Maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 °C and locally even exceeded 1350 °C. This led to comprehensive melting of the precursor Archean granulite assemblages (Grt + Bt + Qtz + Pl + Ksp ± Crd ± Opx ± Sil) followed by peritectic crystallization of aluminous alkali feldspar+Crd + Spl ± Crn ± Sil parageneses and the segregation of small, evolved, biotite leucogranite bodies. However, at a distance of c. 6 km from the centre pre‐impact rock features are largely preserved, although partial replacement of garnet by symplectitic coronas of Crd + Opx ± Spl ± Pl and biotite by orthopyroxene indicate that peak temperatures approached 775 ± 50 °C. Thin interstitial moats of K‐feldspar are closely associated with the orthopyroxene coronas; they are interpreted as the remnants of low‐proportion partial melts generated by biotite breakdown. Both the textures and mineral compositional data support reduced equilibration volumes in these rocks, which reflect rapid isobaric cooling following shock heating and exhumation. The high temperatures and strong lateral thermal gradient are consistent with the modelled impact‐induced isotherm pattern for a 200–300 km diameter impact crater.     

滇西来利山花岗岩年代学、地球化学特征及其壳源部分熔融成因

与锡矿有关的花岗岩成因一直是地学界关注的焦点。来利山花岗岩位于云南省腾冲—梁河地区(花岗岩带上),构造上属于腾梁花岗岩带。该花岗岩带总体呈北北东向带状展布,构成冈底斯的南延部分。来利山岩体主要由似斑状黑云母二长花岗岩和中粗粒黑云母正长花岗岩组成。本次工作在前人研究的基础上对来利山花岗岩进行了岩石学、地球化学和年代学研究,获得二长花岗岩(14L-4)锆石LA-ICP-MS U-Pb年龄为(52.34±0.68)Ma(MSWD=1.4,n=26)。地球化学数据显示二长花岗岩相对富铝(Al2O3=14.70%~15.27%)、碱(K2O+Na2O=8.07%~8.50%)、钙(Ca O=1.64%~2.56%)、REE、Ba。铝饱和度(A/CNK)为0.97~1.12,平均为1.04,属偏铝质到弱过铝质岩石。正长花岗岩相对富硅(Si O2=74.57%~76.69%)、Rb,强烈亏损Ba、Sr、P、Ti、Eu,铝饱和度(A/CNK)为1.05~1.18,平均为1.09,属弱过铝质岩石。两类花岗岩系同源岩浆于不同阶段侵位形成,二者在矿物学、地球化学、年代学等方面表现出良好的演化关系,均显示S型花岗岩的特征。其形成与印度大陆和亚洲大陆碰撞过程中地壳物质部分熔融有关。     

南秦岭佛坪麻粒岩的矿物学和地球化学特征及其构造意义

采用电子探针、红外光谱和全岩化学分析方法,对佛坪麻粒岩开展了系统的矿物学和地球化学研究。研究结果表明本区麻粒岩以酸性岩为主,主要组成矿物为长石、石英、石榴子石、紫苏辉石及少量的黑云母和钙质角闪石。岩石富集轻稀土和大离子亲石元素,亏损高场强元素,具有强烈的Nb、Ta负异常,表明源岩在成因上与洋壳的俯冲消减作用有关。结合区域地质和测年资料,我们认为佛坪麻粒岩源岩的形成与勉略古洋壳向南秦岭地块之下的俯冲有关,是中—晚三叠世南秦岭与扬子地块的陆-陆碰撞所导致的麻粒岩相变质作用的产物。     

Experimental and geochemical evidence for derivation of the El Capitan Granite,California, by partial melting of hydrous gabbroic lower crust

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850°C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635–661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486–1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening.     

An interpretation of the 1883 cataclysmic eruption of Krakatau from geochemical studies on the partial melting of granite

Pumice flow from the 1883 Krakatau eruption significantly differs in both mineral and chemical compositions from any other volcanic rocks or ejecta of the Krakatau group, which belong to the tholeiitic series. Lithic fragments of granitic Rock, discovered in the pumice flow, are similar to West Malayan granitic rocks. No other granitic rock occurs throughout the Krakatau group, therefore, we consider that the granitic fragments came from the underlying complex at depths, where they were captured as foreign materials by the magma.It is possible that sialic crustal materials plunged into depths along a peculiar tectonic structure located at the Sunda Strait, which appears to be a sheared portion caused by deformation of the Sunda arc due to differential movement between the Indo-Australian oceanic plate and the Eurasian continental crust. The crustal materials were partially melted and produced a magma of granitic composition. The magma was mixed with or assimilated by an ascending basaltic magma originating probably from the upper mantle. This resulted in a dacitic magma distinctly dominant in silica, alkalis and volatile components, and the 1883 Krakatau eruption, characterized by the pumice flow of dacitic composition, took place.     

地幔中的水与熔融

自二十世纪九十年代起,通过对天然样品的观察、矿物中水的溶解度的实验研究、以及地球物理的观测,人们逐步认识到地球内部以矿物晶格缺陷形式蕴含着至少与现今海洋水量相当的水。地球内部的水除了显著影响矿物的热导率、电导率、流变学、扩散等性质之外,还对不同构造背景下的地幔熔融产生了重要影响。水作为强不相容元素,在地幔发生熔融时会更倾向于进入到熔体相中,从而改变熔体的结构,影响与残留固相之间的平衡。总体上讲,在等温等压条件下,地幔中水的存在可以显著降低其固相线并增大熔融程度;而在绝热减压过程中,水的存在则可以加深初始熔融的深度,增大地幔整体熔融的区间,从而降低平均熔融程度。近年来,越来越多的证据表明,水对于从岩石圈-软流圈边界的低程度熔融到地幔超大规模熔融产物（大火成岩省）的形成都具有重要作用。本文综述了不同构造背景下（大洋中脊、岛弧、地幔过渡带、大火成岩省、大陆板内、洋岛等）水对地幔熔融影响的新进展,并提出现有研究的不足和未来的挑战。

     

淡色花岗岩的岩石学和地球化学特征及其成因

淡色花岗岩(leucogranite)是一类高铝高硅碱的酸性侵入岩,主要地球化学特征是:SiO2≥72%,Al2O3≥14%,Na2O+K2O~8.5%,富Rb,亏损Th、Ba、Sr,稀土总量较一般花岗岩低(∑REE=(40~120)×10-6),且表现为中等分异的轻稀土弱富集型,一般具有Eu负异常;Sr-Nd-Pb-O同位素指示其岩浆明显的陆壳来源。淡色花岗岩主要发育于陆壳(俯冲)碰撞加厚带,由逆冲折返的俯冲板片变沉积岩部分经过脱水熔融产生。淡色花岗岩可划分为三种不同的岩石类型:(1)二云母型淡色花岗岩,由变泥质岩(或变硬砂岩)在中地壳水平经黑云母(和/或白云母)脱水熔融产生;(2)电气石型淡色花岗岩,由变泥质岩在较低温度下经白云母脱水熔融产生;(3)石榴子石型淡色花岗岩,由长英质下地壳经黑云母脱水熔融产生。源区残留独居石、磷灰石等富REE矿物是淡色花岗岩亏损REE、Th等元素的原因。源岩为变泥质岩及源区残留钾长石是淡色花岗岩亏损Sr、Ba的主要原因。     

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland,southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70391±4. _Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of from 0.70380 to 0.70430 and _Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured _Nd(0) value of –10.2, ⁸⁷Sr/⁸⁶Sr ratio of 0.73679 and a T ^Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher _Nd values (3.9 to 4.6) and slightly lower ⁸⁷Sr/ ⁸⁶Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (La_N/ Yb_N=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of _Nd values, but uniform initial ⁸⁷Sr/⁸⁶Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.     

What controls partial melting in migmatites?

Abstract The layers of six stromatic migmatites from Northern, Western, and Central Europe display small but systematic chemical and mineralogical differences. At least five of these migmatites do not show any signs of largescale metamorphic differentiation, metasomatism, or segregation of melts. It is concluded, therefore, that the compositional layering observed in most of the investigated migmatites is due to compositional differences inherited from the parent rocks. Almost isochemical partial melting seems to be the most probable process transforming layered paragneisses, metavolcanics, or schists into migmatites.
The formation of neosomes is believed to be caused by higher amounts of partial melts formed due to higher amounts of water moving into these layers. The neosomes have less biotite and more K-feldspar, if K-feldspar is present at all, than the adjacent mesosomes. These differences are small but systematic and seem to control the access of different amounts of water to the various rock portions. Petrographical observations, chemical data, and theoretical considerations indicate a close relationship between rock composition, rock deformation, transport of water, partial melting, and formation of layered migmatites.     

Age and sources of matter for the Kedrovskoe gold deposit,Northern Transbaikal tegion,Republic of Buryatia: Geochronological and isotopic geochemical constraints

The paper presents new geochronological and isotopic geochemical data on gold mineralization of the Kedrovskoe deposit. The deposit is located in the northeastern part of the Transbaikal metallogenic province, Russia’s largest. The Early Permian age (273 ± 4 Ma) of mineralization based on the results of Rb–Sr study of metasomatic rocks is correlated with the age of the final phases of Hercynian magmatism in the Baikal–Muya Foldbelt. The Sr, Nd, and Pb isotopic geochemical characteristics of mineralization show that the host rocks are involved in the formation of the latter. It has been established that ore lead was supplied to the hydrothermal system of the deposit mainly from a geochemical reservoir represented by the Neoproterozoic juvenile continental crust of the Baikal–Muya Foldbelt.     

Pn velocity and partial melting—discussion

The seismologically observed P_n velocity anomalies in the conterminous United States are restricted to the lithosphere, but the observed teleseismic delay-time variations are due principally to the regional variations in the physical state (i.e., thickness of lowvelocity zone and/or percent melt, etc.) of the asthenosphere. The observed low P_n velocity has been attributed to partial melting in the upper mantle, but it is shown that the partial-melting model alone cannot explain the seismologically observed Pn velocities in such an anomalous region as the Basin and Range Province. The present structure of the Basin and Range Province is possibly a result of rifting in the western conterminous United States; under it there may lie a mixed structure of old crust and mantle materials. The low-velocity zone under the Basin and Range Province would then be caused by downward chemical transition from the sub-Moho pyrolitic mantle material into a plagioclase-rich ophiolitic (old oceanic crust and upper mantle) composition and associated meltingand then into a peridotitic composition at the bottom of the lowvelocity zone. This mixed material model, with partial melting, would explain the low P_n velocity and low seismic Q in the region, as well as other geophysical observations.     

广东西淋岗花岗岩原地重熔成因的地质地球化学证据

广东西淋岗岩体出露的地质剖面为研究花岗岩成因提供了大量信息。对该岩体的野外地质调查发现:下白垩统白鹤洞组红层(K1bh)与下伏花岗岩体呈突变接触关系,接触界面之下的花岗岩中含大量红岩碎屑,且红岩碎屑往下逐渐减少。岩石地球化学及矿物学测试结果表明:西淋岗花岗岩的成岩物质,应来自盖层之下包括下白垩统白鹤洞组下部红层在内的岩石地层。岩石熔融和岩浆结晶过程中,分异作用导致元素分布具有明显分带性;花岗岩中的石英是在比较稳定的条件下结晶而成,且在石英结晶过程中,SiO2过饱和度逐渐降低。上述研究结果表明,西淋岗花岗岩应是上地壳岩石原地熔融固结形成,而不是远道而来的“侵入体”。     

The role of subgrain boundaries in partial melting

Evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the Lost Creek Gneiss of the Llano Uplift, central Texas, the Wet Mountains of central Colorado, and the Albany-Fraser Orogen, southwestern Australia. Domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals. Material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt, indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph. Subgrain boundaries consist of arrays of dislocations within a crystal lattice, and unlike fractures would not act as conduits for melt migration. Instead, the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there. Preferential melting in high strain locations may be attributed to strain energy, which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds, and/or the presence of small quantities of water associated with dislocations, which may enhance diffusion rates or locally lower the temperature needed for partial melting.     

喜马拉雅造山带的变质作用与部分熔融

喜马拉雅造山带的核心由高级变质岩系和淡色花岗岩构成,是研究碰撞造山作用和板块构造的天然实验室。本文评述了喜马拉雅造山带变质作用和部分熔融研究取得的新进展和存在的争议,主要内容包括:(1)造山带核部具有"三明治"结构,高级变质和部分熔融的高喜马拉雅系列(GHS)夹持在较低级变质的特提斯喜马拉雅系列(THS)和低喜马拉雅系列(LHS)之中,GHS的变质作用程度具有向上和向下部构造层位降低的特征。高喜马拉雅系列主要由高压麻粒岩相到榴辉岩相的变质岩组成,具有1.2~1.6GPa和700~800℃峰期变质条件,顺时针型变质作用P-T轨迹,其进变质以增温增压为特征,退变质早期为近等温或增温降压过程,晚期为降温降压和近等压降温过程;(2)在造山带西段,紧邻缝合带产出的超高压变质岩具有4.4~4.8GPa和560~760℃的峰期变质条件和顺时针型P-T轨迹,并在退变质中期出现加热过程;(3)尽管造山带的高压和超高压变质岩形成在中、高温条件下,但岩石中的石榴石都保存有明显的主量和微量元素生长成分环带特征;(4)造山带变质核下部发育反转的中、高压型变质序列;(5)在造山带核部,变泥质和长英质麻粒岩的强烈部分熔融主要是增压、增温进变质过程中的白云母和黑云母脱水熔融,和近等温或增温降压过程中的黑云母脱水熔融,可以形成花岗质和英云闪长质熔体。加厚下地壳的高变质温度足以使各种成分岩石(包括基性岩)发生深熔,而不需要外来热源;(6)造山带变质核经历了长期的变质演化过程,其进变质始于~47Ma,峰期变质发生在~25Ma,退变质持续到~15Ma。这些岩石也记录了持续的(超过20Myr)高温变质和部分熔融过程。在造山带西段的超高压变质岩具有~46Ma的峰期变质年龄和~40Ma的退变质年龄,所以经历了一个快速俯冲与折返过程;(7)印度大陆西缘与岛弧的碰撞(造山带西段)和印度大陆东缘与大陆弧的碰撞时间一致,为~50Ma;(8)在造山带西段,印度大陆的深和陡俯冲形成了超高压变质岩;而在造山带中段,印度大陆的平缓俯冲形成了中高压变质岩;(9)构造变质不连续在变质核中广泛存在。多重有序逆冲和无序逆冲导致的岩片叠置控制着造山带的地壳结构;(10)现有的构造模型,包括楔形挤出、隧道流、临界楔和构造楔模型,都不能全面合理地解释造山带变质核部的折返机制。