Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A1₂O₃-Fe₂O₃-SiO₂-H₂O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the X_Fe³⁺ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe³⁺ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-X_Fe³⁺ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H₂O, where the X_Fe³⁺ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-X_Fe³⁺ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.     

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

The 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub-greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote-amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high-pressure zone III to lower grade zones II and I to the west and east. The P – T conditions estimated from zoned amphibole indicate an anticlockwise P – T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high-pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.     

Fossil Association from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges Area, Hubei, South China

Apart from previously reported Small Shelly Fossils (SSFs), a macroscopic fossil assemblage, comprising abundant algae, cone-shaped tubular fossil forms, and probable impressions of a megascopic metazoan, comes from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges area of western Hubei Province, south China. The visible fossils are preserved in thin-laminated siltstone or muddy siltstone intercalated between 8–15 mm-thick carbonate deposits, probably representing sedimentary settings of a constrained local depression in the shallow water carbonate platform during the Early Cambrian Meishucunian Stage. The macroscopic fossil association provides significant fossil evidence about the evolution of life from the late Precambrian to the ‘Cambrian explosion’ interval.     

Middle Archean ocean ridge hydrothermal metamorphism and alteration recorded in the Cleaverville area, Pilbara Craton, Western Australia

A hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub-greenschist facies (zones A and B) to greenschist facies (zone C) under low-pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean-floor metamorphism, at a mid-ocean ridge.
The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO₂ in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca-Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO₂ than present-day ocean-ridge hydrothermal alteration, as an interaction between sea water and oceanic crust.     

The youngest blueschist belt in SW Japan: implication for the exhumation of the Cretaceous Sanbagawa high-P/T metamorphic belt

The tectonic evolution of the Northern Shimanto belt, central Shikoku, Japan, was examined based on petrological and geochronological studies in the Oboke area, where mafic schists of the Kawaguchi Formation contain sodic amphibole (magnesioriebeckite). The peak P–T conditions of metamorphism are estimated as 4–4.5 kbar (15–17 km depth), and 240–270 °C based on available phase equilibria and sodic amphibole compositions. These metamorphic conditions are transitional between blueschist, greenschist and pumpellyite–actinolite facies. Phengite K–Ar ages of 64.8 ± 1.4 and 64.4 ± 1.4 Ma were determined for the mafic schists, and 65.0 ± 1.4, 61.4 ± 1.3 and 63.6 ± 1.4 Ma for the pelitic schists. The metamorphic temperatures in the Oboke area are below the closure temperature of the K–Ar phengite system, so the K–Ar ages date the metamorphic peak in the Northern Shimanto belt. In the broad sense of the definition of blueschist facies, the highest‐grade part of the Northern Shimanto belt belongs to the blueschist facies. Our study and those of others identify the following constraints on the possible mechanism that led to the exhumation of the overlying Sanbagawa belt: (i) the Sanbagawa belt is a thin tectonic slice with a structural thickness of 3–4 km; (ii) within the belt, metamorphic conditions varied from 5 to 25 kbar, and 300 to 800 °C, with the grade of metamorphism decreasing symmetrically upward and downward from a structurally intermediate position; and (iii) the Sanbagawa metamorphic rocks were exhumed from ～60 km depth and emplaced onto the Northern Shimanto metamorphic rocks at 15–17 km depth and 240–270 °C. Integration of these results with those of previous geological studies for the Sanbagawa belt suggests that the most probable exhumation mechanism is wedge extrusion.     

苏鲁地体超高压和非超高压花岗质片麻岩的判别标志——来自锆石中矿物包裹体的证据

中国大陆科学钻探工程预先导孔CCSD－PP1中存在两类不同性质的花岗质片麻岩，它们的岩相学性质，地球化学特征，锆石中的包体矿物组合和分布规律以及阴极发光图像均存在明显的判别，其中第一类花岗质片麻岩中的锆石呈完好的自形晶，不含任何超高压矿物包体，自晶体中心至边缘均具有典型的岩浆结晶生长环带，表明该类花岗质片麻岩未经历超高压变质作用，第二类花岗质征麻岩中的锆石呈半自形晶一它形晶，晶体核部保存典型的岩浆结晶生长环带，且无超高压矿物包体，具有继承性锆石的特点，而在晶体的过渡带及其与边缘的交界部位，普遍存在以柯石英粉 包体矿物，表明第二类花岗质片麻岩曾经历了超高压变质作用，且锆石在此变质过程进一步结晶生长，形成新的生长环带，该项成果对于深入探讨苏鲁超高压变质带的俯冲－折返机制及其与岩浆作用的相互关系以及对中国大陆科学钻探工程的选址和实施有着重要的科学意义。