大陆深俯冲过程中的流体及变质化学地球动力学

俯冲带发生的化学作用导致了广泛的脱气作用和岩浆活动,控制着全球物质循环和大陆地壳的生长,是地球演化的基础.实际上,俯冲带化学作用是板块俯冲过程中流体形成、运移和流体-岩石(矿物)相互作用的结果.     

High-pressure blueschists and eclogites in Hong'an: a framework for addressing the evolution of high- and ultrahigh-pressure rocks in central China

The Hong'an region in the Qinling–Dabie collisional zone in eastern China hosts a series of metamorphic rocks exposing a south-to-north distribution from blueschist/blueschist–greenschist, amphibolite, eclogite (kyanite free) and kyanite–eclogite to coesite–eclogite facies rocks that represent progressively deeper levels of the Mesozoic subduction–collision complex. The Hong'an area is interesting for three reasons: (1) it escaped the thermal and structural overprint imparted on much of the Dabie Mountains during Early Cretaceous intrusion of voluminous granites and granodiorites; (2) the high-pressure (HP) Hong'an eclogites are widely distributed, often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist/blueschist–greenschist rocks exposed to the south; (3) the blueschist/blueschist–greenschist facies rocks are generally better exposed than their equivalents in the southeastern Dabie Mountains and offer some opportunity for simultaneous structural and metamorphic analysis. The Hong'an area HP rocks offer perhaps the closest approximation to a preserved snapshot of Mesozoic pressure–temperature (P–T) conditions attending early subduction–exhumation in the region, and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh-pressure (UHP) rocks. The purpose of this contribution is twofold: (1) to document previously unpublished metamorphic and structural data characterizing these HP sequences and their relative continuity in Hong'an; (2) to incorporate these data with recent geochronologic, structural and paleomagnetic information in the context of protracted, late Paleozoic through Mesozoic subduction, collision and exhumation. Metamorphism and exhumation of some of the southern Hong'an HP sequences appear to have occurred concomitant with oceanic subduction immediately to the west, and thus may have preceded widespread continental subduction/collision. Moreover, all of the HP–UHP sequences in the region were exhumed before the end of collision between the Sino-Korean and Yangtze cratons at ca. 160 Ma. Exhumation of HP–UHP rocks both before and during continental plate collision is neither novel for central China nor for other HP–UHP zones, but is important to take into account when reconstructing the evolution of such orogens.     

An AEM study of garnet clinopyroxenite from the Sulu ultrahigh-pressure terrane: formation mechanisms of oriented ilmenite,spinel, magnetite,amphibole and garnet inclusions in clinopyroxene

Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [1[`1]00 1\bar{1}00 ]<sub>Ilm</sub>//[0[`1]0 0\bar{1}0 ]_Cpx (0001)_Ilm//(100)_Cpx; and [110]_Spl//[0[`1]0 0\bar{1}0 ]<sub>Cpx</sub> ([`1]11 \bar{1}11 )_Spl//(100)_Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness <1 μm) and lamellae (with a thickness = 1–10 μm) in host clinopyroxene of sample RZ-11D were probably results of hydration processes, although amphibole plates could otherwise be interpreted as exsolution products. Temporal relations between mineral inclusions in each sample can be established, and a semi-quantitative P–T path for this garnet clinopyroxenite body was derived accordingly. The present results show that the Hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during UHP metamorphism as suggested by previous studies. This study demonstrates that the crystallographic and temporal/spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks.     

Parageneses and compositions of amphiboles from Franciscan jadeite–glaucophane type facies series metabasites at Cazadero, California

Abstract Sodic amphiboles are common in Franciscan type II and type III metabasites from Cazadero, California. They occur as (1) vein-fillings, (2) overgrowths on relict augites, (3) discrete tiny crystals in the groundmass, and (4) composite crystals with metamorphic Ca–Na pyroxenes in low-grade rocks. They become coarse-grained and show strong preferred orientation in schistose high-grade rocks. In the lowest grade, only riebeckite to crossite appears; with increasing grade, sodic amphibole becomes, first, enriched in glaucophane component, later coexists with actinolite, and finally, at even higher grade, becomes winchite. Actinolite first appears in foliated blueschists of the upper pumpellyite zone. It occurs (1) interlayered on a millimetre scale with glaucophane prisms and (2) as segments of composite amphibole crystals. Actinolite is considered to be in equilibrium with other high-pressure phases on the basis of its restricted occurrence in higher grade rocks, textural and compositional characteristics, and Fe/Mg distribution coefficient between actinolite and chlorite. Detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles. At the highest grade, winchite appears at the expense of the actinolite–glaucophane pair. Compositional characteristics of Franciscan amphiboles from Ward Creek are compared with those of other high P/T facies series. The amphibole trend in terms of major components is very sensitive to the metamorphic field gradient. Na-amphibole appears at lower grade than actinolite along the higher P/T facies series (e.g. Franciscan and New Caledonia), whereas reverse relations occur in the lower P/T facies series (e.g. Sanbagawa and New Zealand). Available data also indicate that at low-temperature conditions, such as those of the blueschist and pumpellyite–actinolite facies, large compositional gaps exist between Ca- and Na-amphiboles, and between actinolite and hornblende, whereas at higher temperatures such as in the epidote–amphibolite, greenschist and eclogite facies, the gaps become very restricted. Common occurrence of both sodic and calcic amphiboles and Ca–Na pyroxene together with albite + quartz in the Ward Creek metabasites and their compositional trends are characteristic of the jadeite–glaucophane type facies series. In New Caledonia blueschists, Ca–Na pyroxenes are also common; Na-amphiboles do not appear alone at low grade in metabasites, instead, Na-amphiboles coexist with Ca-amphiboles throughout the progressive sequence. However, for metabasites of the intermediate pressure facies series, such as those of the Sanbagawa belt, Japan and South Island, New Zealand, Ca–Na pyroxene and glaucophane are not common; sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite, and occur only in Fe₂O₃-rich metabasites. The glaucophane component of Na-amphibole systematically decreases from Ward Creek, New Caledonia, through Sanbagawa to New Zealand. This relation is consistent with estimated pressure decrease employing the geobarometer of Maruyama et al. (1986). Similarly, the decrease in tschermakite content and increase in NaM₄ of Ca-amphiboles from New Zealand, through Sanbagawa to New Caledonia is consistent with the geobarometry of Brown (1977b). Therefore, the difference in compositional trends of amphiboles can be used as a guide for P–T detail within the metamorphic facies series.     

陕甘宁盆地坳陷型湖盆缓坡带三角洲前缘短期基准面旋回与储层成因分析

运用高分辨率层序地层的短期基准面旋回原理,对发育于陕甘宁盆地城华地区长 3油层组缓坡带上的三角洲前缘的储集砂体进行细致分析,按成因将储层砂体详细划分为三大类 6种类型,而这几种类型砂体的成因明显受基准面升降、可容纳空间 (A)和沉积物补给量 (S)之间变化关系两方面的影响。在基准面上升过程中,A/S<1时,以截削式河道砂体为主,A/S >1时,则为完整式河道砂体;在基准面下降过程中,A/S <1时,以孤立式河口坝砂体为主,A/S>1时,则以叠加式河口坝和远砂坝砂体为主;在水下坡折带沉积区,则为复合式坝砂体。基准面升降过程中,短期基准面旋回的规律性变化,影响着不同成因砂体的分布,据此建立了坳陷型湖盆缓坡带三角洲前缘不同砂体类型的成因分布模式     

Clinopyroxene–a mineral telescoped through the processes of blueschist facies metamorphism

Abstract Textural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan-shaped medium-grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span of P and/or T for crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature. The first formed clinopyroxene (Jd₂₀Aug₆₅Ac₁₅) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant low X_Jd content towards acmite. At a composition around Jd₂₀Aug₃₀Ac₅₀, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite-omphacite join and clinopyroxene increases in X_Jd toward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant Aug/Ac ratio of 50/50 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na–Ca pyroxenes, close to the binary join Jd–Ac, occur in the lawsonite- and pumpellyite-zones, ranging from X_Jd= 1.0–0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170–240° C by Taylor & Coleman (1968) for these zones does not support the low-temperature extrapolation of the Jd–Ab–Qz curve by Holland (1980). The estimated metamorphic field gradient indicates an inflection point at 7 kbar, 200° C. Below this, blueschist facies metamorphism proceeded under dominant pressure-increase from 4 to 7 kbar at nearly constant temperature, about 150–200° C, whereas at higher grade recrystallization, above the inflection point, the metamorphic temperature increased from 200 to 350° C at nearly constant pressure, about 7–8 kbar. Such an inflection point suggests the depth of underplating of either seamounts or accretionary packages in a subduction zone.     

Contact Metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

The Upper Triassic Karmutsen Subgroup of northeastern VancouverIsland exhibits a clear-cut contact metamorphism of basalticflows, pillows, and pillow breccias in the aureoles of the CoastRange Intrusions. The thermal effect is superimposed upon prehnite-pumpellyitefacies regional metamorphic rocks. The metamorphism of flowsis nearly isochemical except for H₂O. Aquagene tuffs and pillowbreccias, which have undergone extensive chemical alterationthrough diagenesis and burial metamorphism, involve considerablemetasomatism (gain of Si, Na, and Ca and loss of Fe, Mg, andTi) and approach the original basaltic composition in the contactzone. Two prograde metamorphic zones are recognized: an actinolite-chlorite-epidote-albitezone, about 2600 m wide; and a hornblende-calcic plagioclase(An>20) zone, about 900 m wide. Microprobe analyses indicatethat (1) amphibole increases in Al₂O₃, TiO₂, and Na₂O and chloritein Al₂O₃ with progressive metamorphism; (2) the pistacite contentof epidote decreases from Ps = 30 in the prehnite-pumpellyitezone to Ps = 25 in the epidote-actinolite zone; and (3) theAn content of plagioclase jumps from An₀₀–₀₄ for the outerthermal zone to An ₂₀–₇₀ for the inner zone with a compositionalgap between An₀₄ and An₂₀. The following evidence suggests a low-pressure contact metamorphiccondition and therefore a shallow level of the intrusion: (1)the occurrence of prehnite-actinolite-chlorite and absence ofpumpellyite-actinolite-chlorite in the outer thermal zone; (2)wide compositional gaps in both amphibole and plagioclase acrossthe metamorphic zones; (3) absence of an albite-epidote-hornblendeassemblage in transition zone; and (4) the thickness of thesection (Quatsino-Bonanza) overlying the Karmutsen. Mineralparageneses and inferred phase relations suggest that the transitionfrom the prehnite-pumpellyite zone to the epidoteactinolitezone occurs at about 350 ?C and from the epidote-actinolitezone to the plagioclase-hornblende zone at about 470–500?C at relatively high fo₂ conditions.     

塔城盆地寒潮天气特征分析

通过分析塔城盆地30个寒潮个例，对出现寒潮天气的实况资料进行客观统计，着重分析形成寒潮天气的500hPa高度、850hPa温度、地面高压的季节特征以及积雪深度和云天状况在形成寒潮天气中的作用，从而确立影响塔城盆地寒潮天气的有关指标。给出了寒潮预报的经验方程，并进行个例试报。     

Eclogite varieties and petrotectonic evolution of the northern Guatemala Suture Complex

ABSTRACT

Field and petrologic characteristics of two new eclogite localities within the Guatemala Suture Complex (GSC) north of the Motagua Fault are presented. The Tuncaj Hill locality exposes a coherent body of retrogressed eclogite hundreds of metres long that is associated with serpentinite of the North Motagua Unit. The Tanilar River locality exposes numerous bands and lenses of eclogite hosted in sialic gneisses of the Chuacús Complex. The Tuncaj eclogite has a two-stage prograde evolution containing the peak assemblage Grt + Omp + Ttn + Czo + Zo ± Am, formed at temperatures <720°C. In contrast, eclogites of the Tanilar unit are characterized by the paragenesis Omp + Grt + Rt ± Phg ± Qtz ± Ep giving higher peak conditions of T = 720–830°C and P = 2.1–2.7 GPa, near the stability field of coesite. Previously obtained data and our thermobaric calculations suggest distinct petrotectonic evolutions for the various types of eclogites within the suture. The lawsonite eclogites south of the Motagua Fault were probably produced in a mature Farallon subduction zone during the Early Cretaceous. The northern high-pressure (HP) blocks in serpentinite mélange and coherent amphibolite bodies with eclogite relics were generated by the Early Cretaceous subduction of the proto-Caribbean lithosphere under the Great Caribbean Arc. A continental block, the North American passive margin, reached the arc’s trench in the Campanian and was subducted to ca. 80 km depth, producing the eclogites of the Chuacús Complex. As the slab was delaminated and partially exhumed, the continental Chuacús became tectonically juxtaposed with HP blocks of the proto-Caribbean that had been accreted to the Caribbean plate forming the North Motagua Unit. The juxtaposed group migrated to mid-crustal level and was contemporaneously retrogressed under epidote-amphibolite facies conditions.