岩浆的生成、运移和混合——29届国际地质大会专题简介

介绍了洋中脊玄武岩浆、岛弧岩浆、地幔热柱成因的玄武质岩浆、俯冲带岩浆、长英质岩浆、的生成问题;岩浆运移的理论及制约因素;混合岩浆的成因机制及其有关标志。     

Appinite suites and their genetic relationship with coeval voluminous granitoid batholiths

ABSTRACT

Appinite complexes preserve evidence of mantle processes that produce voluminous granitoid batholiths. These plutonic complexes range from ultramafic to felsic in composition, deep to shallow emplacement, and from Neo-Archean to Recent in age. Appinites are a textural family characterized by idiomorphic hornblende in all lithologies, and spectacular textures including coarse-grained mafic pegmatites, fine-grained ‘salt-and-pepper’ gabbros, as well as planar and linear fabrics. Magmas are bimodal (mafic-felsic) in composition; ultramafic rocks are cumulates, intermediate rocks are hybrids. Their geochemistry is profoundly influenced by a mantle wedge extensively metasomatized by fluids/magmas produced by subduction. Melting of spinel peridotite sub-continental lithospheric mantle (SCLM) produces appinites whose geochemistry is indistinguishable from coeval low-K calc-alkalic arc magmatism. Coeval felsic rocks within appinite complexes and adjacent granitoid batholiths are crustal magmas. When subduction terminates, asthenospheric upwelling (e.g. in a slab window, or in the aftermath of slab failure) induces melting of metasomatized garnet SCLM to produce K-rich sho shonitic magmas enriched in large ionic lithophile and light relative to heavy rare earth elements, whose asthenospheric component can be identified by Sm-Nd isotopic signatures. Coeval late-stage Ba-Sr granitoid magmas have a ‘slab failure’ geochemistry, resemble TTG and adakitic suites, and are formed either by fractionation of an enriched (shoshonitic) mafic magma, or high pressure melting of a meta-basaltic protolith either at the base of the crust or along the upper portion of the subducted slab. Appinite complexes may be the crustal representation of mafic magma that underplated the crust for the duration of arc magmatism. They were preferentially emplaced along fault zones around the periphery of the granitoid batholiths (where their ascent is not blocked by overlying felsic magma), and as enclaves within granitoid batholiths. When subduction ceases, appinite complexes with a more pronounced asthenospheric component are preferentially emplaced along active faults that bound the periphery of the batholiths.     

Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. Somma–Vesuvius volcano, Italy: Geochemical and Sr isotope evidence

New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma–Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation–Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC–AFC) formulation. J. Petrol. 999–1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation–fractional crystallization (EC–AFC) model to magmatic systems. J. Petrol. 1019–1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9–10 km) is a fundamental process controlling magma compositions at Mt. Somma–Vesuvius in the last 8 ky BP. Contamination in the mid- to upper crust occurred repeatedly, after the magma chamber waxed with influx of new mantle- and crustal-derived magmas and fluids, and waned as a result of magma withdrawal and production of large and energetic plinian and subplinian eruptions.     

Discrete proterozoic structural terranes associated with low-P, high-T metamorphism, Anmatjira Range, Arunta Inlier, central Australia: tectonic implications

Structural overprinting relationships indicate that two discrete terranes, Mt. Stafford and Weldon, occur in the Anmatjira Range, northern Arunta Inlier, central Australia. In the Mt. Stafford terrane, early recumbent structures associated with D_1a,_1b deformation are restricted to areas of granulite facies metamorphism and are overprinted by upright, km-scale folds F_1c), which extend into areas of lower metamorphic grade. Structural relationships are simple in the low—grade rocks, but complex and variable in higher grade equivalents. The three deformation events in the Mt. Stafford terrane constitute the first tectonic cycle (D₁-D₂) deformation in the Weldon terrane comprises the second tectonic cycle. The earliest foliation (S_2a) was largely obliterated by the dominant reclined to recumbent mylonitic foliation (S_2b), produced during progressive non-coaxial deformation, with local sheath folds and W- to SW-directed thrusts. Locally, (D_2d) tectonites have been rotated by N—S-trending, upright (F_2c) folds, but the regional upright fold event (F_2d), also evident in the adjacent Reynolds Range, rotated earlier surfaces into shallow-plunging, NW—SE-trending folds that dominate the regional outcrop pattern.The terranes can be separated on structural, metamorphic and isotopic criteria. A high-strain D₂ mylonite zone, produced during W- to SW-directed thrusting, separates the Weldon and Mt. Stafford terranes. 1820 Ma megacrystic granites in the Mt. Stafford terrane intruded high-grade metamorphic rocks that had undergone D_1a and D_1b deformation, but in turn were deformed by S_1c, which provides a minimum age limit for the first structural—metamorphic event. 1760 Ma charnockites in the Weldon terrane were emplaced post-D_2a, and metamorphosed under granulite facies conditions during D_2b, constraining the second tectonic cycle to this period.Each terrane is associated with low-P, high-T metamorphism, characterized by anticlockwise P—T—t paths, with the thermal peaks occurring before or very early in the tectonic cycle. These relations are not compatible with continental-style collision, nor with extensional tectonics as the deformation was compressional. The preferred model involves thickening of previously thinned lithosphere, at a stage significantly after (>50 Ma) the early extensional event. Compression was driven by external forces such as plate convergence, but deformation was largely confined to and around composite granitoid sheets in the mid-crust. The sheets comprise up to 80% of the terranes and induced low-P, high-T metamorphism, including migmatization, thereby markedly reducing the yield strength and accelerating deformation of the country rocks. Mid-crustal ductile shearing and reclined to recumbent folding resulted, followed by upright folding that extended beyond the thermal anomaly. Thus, thermal softening induced by heat-focusing is capable of generating discrete structural terranes characterized by subhorizontal ductile shear in the mid-crust, localized around large granitoid intrusions.     

Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M_1–4, and deformation events, D_1–4, in the period 1820–1590 Ma. The north-western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M₁-D₁, and shows a spectacular lateral transition from muscovite + quartz-bearing schists to interlayered andalusite-bearing migmatites and two-pyroxene granofelses that reflect extremely low-pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K-feldspar + quartz-bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M₁ is inferred from syn- to late-D₁ sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M₁ have been variably overprinted to the south-east by a c. 1760 Ma M₂–D₂ event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D₂, and metapelites that have M₁ andalusite and K-feldspar overprinted by M₂ sillimanite and muscovite. The south-eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M₂–D₂, with metapelitic gneisses characterized by spinel + sillimanite + K-feldspar + quartz-bearing assemblages that suggest peak M₂ conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D₂ occurred during essentially isobaric cooling. A third granulite facies event, M₃, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M₄, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M_1–3 assemblages. The superimposed effects of M_1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.     

Mantle-Crust Interaction in Petrogenesis of the Gabbro-Granite Association in the Preobrazhenka Intrusion,Eastern Kazakhstan

The paper reports results of petrological-geochemical, isotope, and geochronological studies of the Preobrazhenka gabbro–granitoid massif located in the Altai collisional system of Hercynides, Eastern Kazakhstan. The massif shows evidence for the interaction of compositionally contrasting magmas during its emplacement. Mineralogical–petrological and geochemical studies indicate that the gabbroid rocks of the massif were formed through differentiation of primary trachybasaltic magma and its interaction with crustal anatectic melts. Origin of the granitoid rocks is related to melting of crustal protoliths under the thermal effect of mafic melts. The mantle–crust interaction occurred in several stages and at different depths. A model proposed here to explain the intrusion formation suggests subsequent emplacement of basite magmas in lithosphere and their cooling, melting of crustal protolith, emplacement at the upper crustal levels and cooling of the granitoid and basite magmas. It was concluded that the formation of gabbro-granitoid intrusive massifs serves as an indicator of active mantle–crust interaction at the late evolutionary stages of accretionary–collisional belts, when strike-slip pull-apart deformations causes the high permeability of lithosphere.     

Constraints on the depth of generation and emplacement of a magmatic epidote‐bearing quartz diorite pluton in the Coast Plutonic Complex,British Columbia

Petrology and P–T estimates indicate that a magmatic epidote‐bearing quartz diorite pluton from Mt. Gamsby, Coast Plutonic Complex, British Columbia, was sourced at pressures below ～1.4 GPa and cooled nearly isobarically at ～0.9 GPa. The P–T path indicates that the magma was within the stability field of magmatic epidote early and remained there upon final crystallization. The pluton formed and crystallized at depths greater than ～30 km. REE data indicate that garnet was absent in the melting region and did not fractionate during crystallization. This suggests that the crust was less than or equal to ～55 km thick at 188 Ma during the early phases of magmatism in the Coast Plutonic Complex. Late Cretaceous contractional deformation and early Tertiary extension exhumed the rocks to upper crustal levels. Textures of magmatic epidote and other magmatic phases, combined with REE data, can be important for constraining the P–T path followed by magmas.     

The Origin of the Mesozoic Associated Multiseries Granitoid Magma in the Shandong Peninsula

Three melting events of the earth's crust occurred during the period of 220-120 Ma in the Shandong Pe-ninsula. Three subcycles of granitoid magma including six rock series were generated in the faulted granitoidmagma belt. The parent magma of several rock series formed earliest originated from the lower crust ofgranulite facies; following the increase of geothermal temperature the source magma would migrate into themiddle crust of amphibolite facies. In the diapiric granitoid magma belt, the granitoid magma was formed firstin granitic layer of the upper crust, and then in the middle crust. In each subcycle the generation of magmastarted with the generation of more mafic one and finished with low eutectic one; they were formed in the formof layered melting in a particular position of the crust.     

胶辽吉地区古元古代早期花岗岩的侵位模式及其与隆滑构造的关系

通过对胶辽吉地区古元古代裂谷带的深入研究，提出了伸展构造环境下中深部地壳花岗质岩浆侵位的一种模式，即花岗质岩浆沿基底与盖层之间的拆离滑脱带多次贯入，形成岩席，之后随着地壳拉伸（或伸展）作用，这些岩席逐渐远离侵位中心，发生侧向迁移。这种大陆壳内中深部花岗质岩浆的侵位和地壳的侧向伸展的模式，与有些学者提出的大洋中脊玄武质岩浆的侵位和洋壳扩张的模式是基本相似的。这一模式也是大陆地壳快速生长加厚的一种有效机制。由于花岗质岩浆的上涌引起上覆盖层因重力失稳形成一系列顺层滑脱构造组合。它们与花岗岩和基底共同构成中深构造层次岩浆隆起－顺层分层滑脱构造系     

Petrogenesis of ultramafic rocks in the Vammala Nickel Belt: Implications for crustal evolution of the early Proterozoic Svecofennian arc terrane

Ultramafic rocks occur within polydeformed high grade metasedimentary rocks throughout the Vammala Nickel Belt (Fennoscandian Shield) and have been subdivided into cumulate-textured intrusive bodies and high-Mg (picritic) eruptive units.

The crystallisation sequence olivine(+cotectic chromite)-orthopyroxene-clinopyroxene-amphibole, the decreased stability field of olivine and fractionation of relatively Al-rich pyroxenes and chromite all suggest that the cumulate-textured intrusions crystallised from a relatively hydrous parental magma at moderate crustal pressures. The parental magma was enriched in Th, LILE and LREE, being similar to modern tholeiitic arc basalts. Trace element ratios suggest that Svecofennian turbidites were the ultimate source of this enriched component. Although the mechanism of contamination is inconsistent with simultaneous assimilation and fractional crystallisation in situ, this can be explained by a model in which magma flowed through subvertical conduits and assimilated adjacent crust.

Characteristics of metamorphism, deformation and subsolidus equilibration and age determinations all suggest that emplacement of cumulate-textured bodies coincided with the peak of ongoing regional metamorphism and deformation. The cumulate-textured intrusions are interpreted as representing middle crustal expressions of early Proterozoic Svecofennian arc magmatism. This model also predicts that the same magmatism resulted in the emplacement of more evolved arc plutonic complexes at higher crustal levels (that have since been eroded), and the formation of large ultramafic cumulate complexes in the lower crust and at the crust-mantle boundary region, and may thus assist in interpreting deep seismic and gravimetric anomalies within Svecofennian domain.

In contrast, the metapicritic supracrustal rocks have trace element signatures similar to transitional MORB, and cannot represent magmas complementary to the cumulate-textured intrusions. The metapicrites represent supracrustal formations which are clearly older than the synorogenic cumulate-textured intrusions, and their spatial association is coincidental only.     

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7085) and lower ε_Nd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.     

New insights into the magmatic plumbing system of the South Australian Quaternary Basalt province from 3D seismic and geochemical data

Recent seismic reflection studies of large-volume, anorogenic basaltic provinces at passive continental margins have challenged the traditional viewpoint that erupted magmas predominantly ascend through the lithosphere via dykes that exploit high-angle faults. However, such seismic-based methods are yet to be applied to identify the magmatic plumbing systems of low-volume basaltic provinces, such as the Cretaceous–Cenozoic Newer Volcanic Province (NVP) in southeastern Australia (total volume <20 000 km³). The South Australian Quaternary Basalt (SAQB) represents the most recent phase of activity both within the NVP and on the Australian continent. This province is located within the Otway Basin, and a large amount of seismic data from gas exploration is available for this region. Consequently, the SAQB represents a superb natural laboratory in which geochemical and seismic data can be combined in order to study the magmatic plumbing system of a low-volume continental basalt province and discriminate between the competing hypotheses for magma transport through the Earth's crust. Geochemical analyses and thermodynamic modelling suggests that the magma that fed the SAQB was generated by adiabatic decompressional melting of a secondary mantle plume. These models imply that melt segregation took place at successively lower pressures from about ～4000 to 3000 MPa during the ca 1 Ma SAQB eruptive history that culminated in the Northern Group, Mt Schank and Mt Gambier eruptions. During ascent, the magma underwent 34–41% fractional crystallisation and cooled ～200 °C, while residing in the crust for a time period on the order of days to weeks. Interpretation of a 3D seismic survey that overlaps with the northeastern part of the SAQB reveals a saucer-shaped sill with an unusual morphology, exhibiting a series of vertical concentric steps towards its outer rim. This sill appears to be fed by magma that intruded along steep normal faults from a feeder dyke that is hosted by the regional, NW–SE-trending Hungerford-Kalangadoo Fault. Our results suggest that the melt that fed the SAQB rose through the crust via dykes and high-angle normal faults, with less evidence for significant horizontal transport of magma than observed in large-volume basaltic provinces in sedimentary basins at rifted continental margins, possibly highlighting a correlation between the extent, volume and magma supply rate within a basaltic province and the nature of magma ascent.     

On the role of diapirism in the, segregation, ascent and final emplacement of granitoid magmas

Only bodies of magma with a high crystal content and partially molten (crustal) country rocks can ascend as diapirs; once such an envelope is pierced, the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole. Deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content, which may then continue ascent via fractures and dykes.

The details of the mechanisms of granitoid magma segregation are still unknown, but it appears that many magmas hegin their ascent through the crust as mushes with at least 50% melt, and that such magmas are rheologically able to ascend through a thickness of crust. This ascent mechanism explains the dearth of structures attributable to the ascent of granitoids, in contrast to the abundance of structures that developed during their final emplacement.

When a magma becomes too crystalline (melt < 25%) to continue its ascent via dykes, it is immobilised. At approximately this stage, a hydrous magma may become saturated with water and release fluids into the aureole, making it particularly susceptible to deformation. Magma that continues to arrive at this level is also immobilised, and the pluton grows as a ballooning diapir. These characteristically deform themselves and their aureoles by bulk shortening.

Magmas that are able to ascend to shallow depths, largely by virtue of lower water contents and higher initial temperatures, tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence. High level intrusions lend to be tabular, are also fed by dykes or conduits, and assemble in tabular batholiths.     

揭开南岭地壳形成演化之谜

南岭的形成演化目前尚不清楚,作为板内造山的一个典型,研究南岭地区的地壳构造对大陆动力学有重要意义。本文对南岭地区重力异常进行了多尺度密度反演,首先利用小波变换对重力异常进行多尺度分解,接着利用功率谱分析方法估算各等效层场源的平均深度,然后利用广义线性反演方法进行各层密度反演,取得区域地壳多个深度上的密度扰动图像。用小波变换多尺度分析和三维密度结构反演确定了南岭中上地壳存在独立的构造单元,它具有低密度性质,反映区域规模的大花岗岩基。南岭中上地壳的构造单元位于北纬24°～26°,东经100°～116°,深达22 km左右。深度22 km以下南岭地壳低密度带与武夷低密度带连通为武夷—云开构造带。南岭最终形成与100 Ma以前特提斯洋俯冲和亚欧板块与加里曼丹地体的陆岛碰撞直接有关。同期发生的区域规模的燕山晚期南岭花岗岩基对应特提斯洋向华南俯冲的第二岩浆带。基于地球物理资料提出的这个陆岛俯冲碰撞假说能否成立,还需要更多岩石学的直接证据证实。     

An overview of anorthosite-bearing layered intrusions in the Archaean craton of southern West Greenland and the Superior Province of Canada: implications for Archaean tectonics and the origin of megacrystic plagioclase

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An₇₀) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth.     

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane,central Appalachian Piedmont Province

ABSTRACT

The Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.     

Bulk arc strain,crustal thickening,magma emplacement,and mass balances in the Mesozoic Sierra Nevada arc

Quantifying crustal deformation is important for evaluating mass balance, material transfer, and the interplay between tectonism and magmatism in continental arcs. We present a dataset of >650 finite strain analyses compiled from published works and our own studies with associated structural, geochronologic, and geobarometric information in central and southern Sierra Nevada, California, to quantify the arc crust deformation. Our results show that Mesozoic tectonism results in 65% arc-perpendicular bulk crust shortening under a more or less plane strain condition. Mesozoic arc magmatism replaced ∼80% of this actively deforming arc crust with plutons requiring significantly greater crustal thickening. We suggest that by ∼85 Ma, the arc crust thickness was ∼80 km with a 30-km-thick arc root, resulting in a ∼5 km elevation. Most tectonic shortening and magma emplacement must be accommodated by downward displacements of crustal materials into growing crustal roots at the estimated downward transfer rate of 2–13 km/Myr. The downward transfer of crustal materials must occur in active magma channels, or in “escape channels” in between solidified plutons that decrease in size with time and depth resulting in an increase in the intensity of constrictional strain with depth. We argue that both tectonism and magmatism control the thickness of the crust and surface elevation with slight modification by surface erosion. The downward transported crustal materials initially fertilize the MASH zone thus enhancing to the generation of additional magmas. As the crustal root grows it may potentially pinch out and cool the mantle wedge and thus cause reduction of arc magmatism.     

http://www.sciencedirect.com/science/article/pii/S1674987113000881

Granitod batholiths of I-type features(mostly granodiorites and tonalites),and particularly those forming the large plutonic associations of active continental margins and intracontinental collisional belts,represent the most outstanding magmatic episodes occurred in the continental crust.The origin of magmas,however,remains controversial.The application of principles from phase equilibria is crucial to understand the problem of granitoid magma generation.An adequate comparison between rock compositions and experimental liquids has been addressed by using a projected compositional space in the plane F(Fe t Mg)eAnorthiteeOrthoclase.Many calc-alkaline granitoid trends can be considered cotectic liquids.Assimilation of country rocks and other not-cotectic processes are identifed in the projected diagram.The identifcation of cotectic patterns in batholith implies high temperatures of magma segregation and fractionation(or partial melting)from an intermediate(andesitic)source.The comparison of batholiths with lower crust granulites,in terms of major-element geochemistry,yields that both represent liquids and solid residues respectively from a common andesitic system.This is compatible with magmas being formed by melting,and eventual reaction with the peridotite mantle,of subducted mélanges that are fnally relaminated as magmas to the lower crust.Thus,the off-crust generation of granitoids batholiths constitutes a new paradigm in which important geological implications can be satisfactorily explained.Geochemical features of Cordilleran-type batholiths are totally compatible with this new conception.     

Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks,Southwest Japan Arc

Cretaceous-Paleogene granitoid rocks and contemporaneous volcanic rocks are widely distributed in the Inner Zone of Southwest Japan. This intense intermediate to felsic magmatism is considered to have taken place on the eastern margin of the Eurasian Continent, before the Southwest Japan Arc drifted away from the continent in the middle Miocene, resulting in the opening of the Japan Sea. The granitoid rocks show regional variations in terms of their radiometric age, petrography, Sr, Nd and O isotope ratios. Based on Sr and Nd isotope ratios, granitoid rocks can be divided into three zones (South, Transitional and North) between the Median Tectonic Line and the Japan Sea. Granitoid rocks and associated gabbros of the North Zone have low initial Sr isotope ratios (0.7048 to 0.7068) and high initial Nd values (+3 to-2.2), whereas granitoid rocks and gabbros from the South Zone have high initial Sr isotope ratios (0.7070 to 0.7088) and low initial Nd values (-3.0to-8.0). Most granitoid rocks from the Transitional Zone have Sr and Nd isotope ratios that lie between those of the North and South Zones, although there is some overlap. Contamination of magmas by upper crust cannot explain this geographical variation in Sr and Nd isotopes. Instead, the regional variation is attributed to compositionally different, magma sources (probably upper mantle and lower crust), beneath the North and South Zones. This is supported by the Sr and Nd isotopic ratios of upper mantle and lower crustal xenoliths included in Cenozoic volcanic rocks in the North and South Zones. These ratios are similar to those of the granitoid rocks in the respective zones. It is suggested that a micro-continent or island arc consisting of continental crust was underthrust beneath the South Zone before or during the Cretaceous, resulting in compositionally distinct sources for granitoid rocks of the North and South Zones. The large variation observed in Sr and Nd isotope ratios for Transitional Zone granitoid rocks is explained by variable proportions of the two different crustal and upper mantle components.     

东昆仑造山带花岗岩及地壳生长

东昆仑造山带是青藏高原内可与冈底斯相媲美的又一条巨型构造岩浆岩带。该带内的花岗岩形成可以划分为4个时段,分别与4个造山旋回相对应：前寒武纪（元古宙）;早古生代;晚古生代—早中生代;晚中生代—新生代。其中,以晚古生代—早中生代（或称华力西—印支旋回）、特别是三叠纪的花岗岩最为发育。东昆仑造山带基底主要形成于古元古代晚期。其早古生代构造-岩浆事件序列与北祁连造山带可以对比,属祁连—东昆仑加里东造山系统的一部分。到晚古生代—早中生代时东昆仑卷入古特提斯构造体制,属于古特提斯造山系统的北缘。华力西—印支是一个完整的造山旋回,与西南“三江”古特提斯的演化历史相似。昆南缝合带是当时中国南北大陆的主要构造分界线。新生代印度—欧亚大陆的碰撞,使东昆仑造山带又卷入了青藏大陆碰撞造山系统,但对东昆仑的影响是一种远程效应。 　　东昆仑造山带大陆地壳主要形成于古元古代晚期,但在显生宙还有新生地壳 （juvenile crust） 产生,与兴蒙、冈底斯、安第斯等造山带相似。东昆仑花岗岩带中丰富的幔源岩浆底侵作用与壳-幔源岩浆混合作用的证据,以及花岗岩类的Nd、Sr同位素成份（87Sr/ 86Sr初始值多数小于0.710;εNd（t ）值变化于－9.2和＋3.6之间）,说明 地幔物质的注入及其与地壳物质的混合,对显生宙地壳的形成演化起着重要作用,是显生宙东昆仑地壳生长的重要方式。根据花岗质寄主岩、镁铁质暗色微粒包体（MME）及底侵辉长岩的锆石SHRIMP U-Pb定年,东昆仑造山带在显生宙发生过两次大规模的底侵作用与岩浆混合作用,一次在早-中泥盆世（394~403 Ma）,另一次在中三叠世（239～242 Ma）,分别相当于加里东旋回、华力西-印支旋回的俯冲结束/碰撞开始阶段。