Shape, size, spatial distribution and composition of garnet crystals in highly deformed gneiss of the Otter Lake area, Québec, and a model for garnet crystallization

Garnet–biotite–(sillimanite) gneiss (～700 °C, 7 kbar) of the Otter Lake area in the Western Grenville Province (Canadian Shield) occurs as granitic gneiss (group 4) that forms a large part of the Otter Complex, and as widely distributed, more heterogenous metasedimentary gneiss (group 2). In one sample of group 4 gneiss (Qtz₂₅ Pl₃₄ Kfs₂₈ Bt₁₀ Grt_2.5 Sil₁) the true diameter (determined by serial grinding) of subhedral garnet crystals ranges from 0.2 to 3.0 mm, with a mode at 1.0 mm. Nearest‐neighbour measurements in this sample, and in surfaces of nine additional samples (all <5% garnet) confirm that garnet crystals are distributed mainly at random; slight clustering was detected in two samples. In one sample of group 4 gneiss, microprobe analyses on sections through crystal centres (obtained by serial slicing), reveal that small crystals and margins to large crystals contain more Fe and Mn and less Mg than the broad central regions of large crystals. Based on these and previous results, together with theoretical considerations, a crystallization model is proposed, in which, (i) garnet was produced by the continuous reaction, Ms + Bt + Qtz → Grt + Kfs + H₂O, (ii) nucleation occurred by the random selection of randomly distributed Ms–Bt–Qtz triple junctions, (iii) the rate of linear growth remained constant, and (iv) as temperature increased, the rate of nucleation first increased slowly, then remained nearly constant, and finally declined. Within‐population compositional homogenization was followed, on cooling, by local Fe–Mg–Mn exchange with biotite.     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.     

Remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the Dabie-Sulu terranes, eastern China

A combined oxygen‐isotope and fluid‐inclusion study has been carried out on high‐ and ultrahigh‐pressure metamorphic (HP/UHPM) eclogites and garnet clinopyroxenite from the Dabie‐Sulu terranes in eastern China. Coesite‐bearing eclogites/garnet clinopyroxenite and quartz eclogites have a wide range in whole‐rock δ¹⁸O_VSMOW, from 0 to 11‰. The high‐T oxygen‐isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation, and the wide range in whole‐rock oxygen‐isotope composition is thought to be a presubduction signature of the precursors. Aqueous fluids with variable salinities and gas species (N₂‐, CO₂‐, or CH₄‐rich), are trapped as primary inclusions in garnet, omphacite and epidote, and in quartz blebs enclosed within eclogitic minerals. In high‐δ¹⁸O HP/UHPM rocks from Hujialin and Shima, high‐salinity brine and/or N₂ inclusions occur in garnet porphyroblasts, which also contain inclusions of coesite, Cl‐rich blue amphibole and dolomite. In contrast, in low‐δ¹⁸O eclogites from Qinglongshan and Huangzhen, the Cl concentrations in amphibole are very low, < 0.2 wt.%, and low‐salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores. These low‐salinity fluid inclusions are believed to be remnants of meteoric water, although the fluid composition was modified during pre‐ and syn‐peak HP/UHPM. Eclogites at Houshuichegou and Hetang contain CH₄‐rich fluid inclusions, coexisting with high‐salinity brine inclusions. Methane was probably formed under the influence of CO₂‐rich aqueous fluids during serpentinisation of mantle‐derived peridotites prior to or during plate subduction. Remnants of premetamorphic low‐ to high‐salinity aqueous fluid with minor N₂ and/or other gas species preserved in the Dabie‐Sulu HP/UHPM eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors, implying very limited fluid–rock interaction during syn‐ and post‐peak HP/UHPM.     

P–T–t evolution of garnet amphibolites in the Wutai–Hengshan area,North China Craton: insights from phase equilibria and geochronology

Garnet amphibolites can provide valuable insights into geological processes of orogenic belts, but their metamorphic evolution is still poorly constrained. Garnet amphibolites from the Wutai–Hengshan area of the North China Craton mainly consist of garnet, hornblende, plagioclase, quartz, rutile and ilmenite, with or without titanite and epidote. Four samples selected in a south–north profile were studied by the pseudosection approach in order to elucidate the characteristics of their metamorphic evolution, and to better reveal the northwards prograde change in P–T conditions as established previously. For the sample from the lower Wutai Subgroup, garnet exhibits obvious two‐substage growth zoning characteristic of pyrope (X_py) increasing but grossular (X_gr) decreasing outwards in the core, and both X_py and X_gr increasing outwards in the rim. Phase modelling using thermocalc suggests that the garnet cores were formed by chlorite breakdown over 7–9 kbar at 530–600 °C, and rims grew from hornblende and epidote breakdown over 9.5–11.5 kbar at 600–670 °C. The isopleths of the minimum An in plagioclase and maximum X_py in garnet were used to constrain the peak P–T conditions of ~11.5 kbar/670 °C. The modelled peak assemblage garnet + hornblende + epidote+ plagioclase + rutile + quartz matches well the observed one. Plagioclase–hornblende coronae around garnet indicate post‐peak decompression and fluid ingress. For the samples from the south Hengshan Complex, the garnet zoning weaken gradually, reflecting modifications during decompression of the rocks. Using the same approach, the rocks are inferred to have suprasolidus peak conditions, increasing northwards from 11.5 kbar/745 °C, 12.5 kbar/780 °C to 13 kbar/800 °C. Their modelled peak assemblages involve diopside, garnet, hornblende, plagioclase, rutile and quartz, yet diopside is not observed petrographically. The post‐peak decompression is characterized by diopside + garnet + quartz + melt = hornblende + plagioclase, causing the diopside consumption and garnet compositions to be largely modified. Thus, the pesudosection approach is expected to provide better pressure results than conventional thermobarometry, because the later approach cannot be applied with confidence to rocks with multi‐generation assemblages. U–Pb dating of zircon in the Wutai sample records a protolith age of c. 2.50 Ga, and a metamorphic age of c. 1.95 Ga, while zircon in the Hengshan samples records metamorphic ages of c. 1.92 Ga. The c. 1.95 Ga is interpreted to represent the pre‐peak or peak metamorphic stages, and the ages of c. 1.92 Ga are assigned to represent the cooling stages. All rocks in the Wutai–Hengshan area share similar clockwise P–T morphologies. They may represent metamorphic products at different crustal depths in one orogenic event, which included a main thickening stage at c. 1.95 Ga followed by a prolonged uplift and cooling after 1.92 Ga.     

鄂陵湖晚全新世沉积物记录的黄河源区气候环境变化

通过对青藏高原黄河源区鄂陵湖35cm长的沉积物岩芯（NR-1）中总有机碳含量（TOC）、总氮含量（TN）、碳氮比值（C_org/N_tot）以及有机碳同位素（δ¹³C_org）和总氮同位素（δ¹⁵N_tot）等地球化学指标的分析,并结合青海地区文献资料记载的人口数量变化以及高原湖泊水位记录,对研究区近1500a以来的气候环境变化进行了深入探讨。结果表明：近两百多年以来鄂陵湖沉积物中TOC和TN指标呈现明显上升的趋势,与青海东部地区人口数量变化趋势一致,表明近代人类活动已经对青藏高原最大淡水湖生态系统产生了显著的影响;而沉积物中δ¹³C_org值变化与青海湖水文环境重建指标具有可比性,说明高原大型淡水湖泊沉积物δ¹³C_org值可指示湖泊水位波动;此外δ¹⁵N_tot值变化趋势与青藏高原东北部已有的古温度记录总体一致,推测鄂陵湖沉积物δ¹⁵N_tot值可能响应了湖泊水体营养状态的变化,间接指示了湖泊夏季表层水温的波动。本研究为青藏高原腹地高海拔现代湖泊沉积物中有机碳和总氮含量及其同位素指标的解译提供了全新的思路。

     

邦铺钼(铜)矿床二长花岗斑岩、黑云二长花岗岩锆石Hf同位素和Ce4+/Ce3+比值

文章对邦铺钼(铜)矿区产出的中新世二长花岗斑岩[(16.23±0.19)Ma]及古新世黑云二长花岗岩[(62.1±1.9)Ma]的锆石进行了微量元素和Hf同位素组成的研究.锆石Hf原位分析表明,邦铺钼(铜)矿区含矿二长花岗斑岩、成矿前黑云二长花岗岩的176Hf/177Hf比值分别为0.282 818～0.282 904...