东昆仑得尔龙地区华力西期花岗岩地质地球化学特征及构造环境分析

东昆仑得尔龙地区花岗岩体侵位于二叠纪—三叠纪早期,岩石类型为黑云母花岗闪长岩、黑云母二长花岗岩、二云母二长花岗岩。早期次的黑云母花岗闪长岩中含有暗色的镁铁质矿物包体。SiO2含量为65．04％～73．47％,全碱含量为5．29％～8．52％,K2 O／Na2 O 平均值为0．70,Al2 O3平均为14．79％;亏损高场强元素 Ta, Nb;∑REE平均为142．9＆#215;106,轻稀土元素相对富集,（La／Yb）N 平均为17．15,δEu 平均为0．71,表现为弱亏损。研究表明得尔龙地区花岗岩属次铝过铝（高钾）钙碱性 S 型花岗岩,形成于后造山环境;岩浆源区的物质是多源的,主要为地壳物质的重熔,其次为幔源岩浆的底侵。     

Mineralogical and geochemical characteristics of scheelite-bearing skarns,and genetic relations between skarn mineralization and petrogenesis of the associated granitoid pluton at Sargipali,Sundergarh District,Eastern India

Skarn rocks occur at the contact between calcite-bearing dolomitic marbles and granitoids (massive varieties with pegmatites) in close spatial association with the mica schist-hosted Proterozoic Pb–Cu–Ag sulfide deposits at Sargipali, Sundergarh District, Eastern India. The exoskarn (pyroxene–garnet) of variable width (1 to 30 m) occurs in marble proximal to the granitic intrusion, and endoskarn (pyroxene–epidote) is variably developed (< 1 to 10 m). Molybdenum-free scheelite with minor pyrrhotite (0.2%) is found only in late garnet–clinopyroxene exoskarn assemblages.     

中亚造山带天河石花岗岩及相关铷矿床的主要特征与研究进展

铷是重要的"关键金属"矿产资源,是未来各国资源争夺的焦点。虽然我国铷矿资源总量丰富,但主要为低品位难以加工利用的花岗岩型铷矿床,而以铁锂云母、锂云母和铯沸石等作为矿石矿物的高品位易加工花岗伟晶岩型铷矿床非常有限。因此,富铷花岗岩及相关铷矿床的形成过程、元素分异机制以及铷在不同矿物相中的赋存状态和控制因素是铷矿床成矿机制研究和找矿工作的关键。本文在对花岗(伟晶)岩铷矿主要研究进展进行综述的基础上,简介中亚造山带东、西段典型天河石花岗岩及相关铷等稀有金属矿床的主要特征和时空分布,并对未来研究重点进行了展望。本文认为,中亚造山带是全球最重要的天河石花岗岩和相关稀有金属矿床成矿域,其西段大量发育三叠纪天河石花岗岩,而东段大量发育晚侏罗至早白垩世天河石花岗岩。两者形成时代和构造背景分别与古亚洲洋向古特提斯洋构造域,以及古亚洲洋向古太平洋构造域的巨大转折相对应,铷等稀有金属成矿潜力巨大,值得开展深入的年代学、岩石学和矿床成因研究。     

Are South China granites special in forming ion-adsorption REE deposits?

Ion-adsorption REE deposits associated with clay minerals are the main global HREE producer. The majority of these deposits are formed by the weathering of granites in South China, but whether there is any fundamental difference between the granites in and outside South China is still unclear. Besides, an effective evaluation system of granite mineralization potential is urgently needed for HREE exploration.To answer this question, we compiled a global granite geochemical dataset from within (n = 1932) and outside (n = 6109) South China, together with a dataset of representative REE deposits in South China (n = 128). The geochemical comparation shows that the South China granites share similar REE contents with those of many granites from places outside South China. Such similarity has also been found between REE ore-related and ore-barren granites in South China. This shows that granites from outside South China could also have ore-forming potential. Warm humid climate and quasi-equalized crustal state promote chemical weathering to continuously leach REEs and store them in the weathering crust. The enrichment ratio (Rx) can be used to quantify the climatic effect between orebodies and parent rocks. The calculated average Enrichment Ratios (Rx) of LREE- and HREE-rich deposits are 2.41 and 2.68, respectively. Sufficient REE content in granite is the prerequisite for mineralization, and we propose that the combination of the minimum REE + Y (172 and 108 ppm in LREE- and HREE-rich parent rocks, respectively) and REE oxide ratio (1.32) can reveal the granite metallogenic potential. Together with the suitable tropical and temperate climate area with ion-adsorption REE deposits, we further identified certain regions with high REE mineralization potential outside South China to assist future exploration.     

Petrology of nepheline syenite gneiss from Amazonian Brazil

Nepheline syenite gneiss is exposed in three localities near the village of Boca Nova (47·04′W, 1°51′S) in northeastern Pará, Brazil. Isotopic and chemical evidence for an igneous premetamorphic history include: (1) bulk compositions which plot in the Q–Ne–Ks system on a peralkaline fractionation curve; (2) bulk compositions which fall on the Hviddal igneous trend in the SiO₂–Al₂O₃–Na₂O + K₂O molecular diagram; (3) an average K/Rb ratio identical to the average for nepheline syenites of the Khibina massif; (4) K/Rb ratios which lie on the ideal igneous fractionation curve; (5) Nb(Ta) contents far in excess of average crustal abundance; and (6) a low Sr₈₇/Sr⁸⁶ value (0·7034) characteristic of mantle derivation. Post-kinematic lit-par-lit injections, discordant veins, and concordant pods of leucocratic nepheline–alkali feldspar pegmatite partially replaced the metamorphic assemblage and destroyed any microscopic equilibrium texture that may have been present. The final disequilibrium assemblage consists of Fe-rich biotite and two generations each of sodic plagioclase, microcline, and nepheline. Some minor metasomatic chemical effects may be present in the rocks but a case for nephelinization of crustal rocks cannot be documented. Field, petrographic, and chemical constraints indicate that the Boca Nova gneiss is an early-stage migmatite formed by anatexis of metamorphosed igneous nepheline syenite or its volcanic equivalent. Regional metamorphism and partial melting opened the Rb-Sr system so that the whole-rock Rb/Sr isochron age of 724 m.y. has no unambiguous interpretation. A K/Ar age of 580 ± 10 m.y. from biotite denning the metamorphic foliation is regarded as a minimum date for the regional metamorphism and anatexis. No other alkaline rocks of comparable age are known in the Sao Luis Craton.     

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

In the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh-pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well-preserved phengite-bearing eclogite sample from the Nordfjord region. The sample was investigated using phase-equilibrium modelling, trace-element analyses of garnet, trace- and major-element thermobarometry and quartz-in-garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near-isobaric heating to 660–680°C. Near-isothermal decompression to 10–14 kbar is recorded in fine-grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end-member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.     

皖南姚村岩体花岗岩风化壳稀土元素赋存特征

皖南地区花岗岩风化壳中稀土元素普遍富集，局部已成为矿床，其中，郎溪县姚村岩体风化壳富集程度较高。LA- ICP- MS锆石U- Pb定年表明，姚村花岗岩体的形成年龄为127.9±1.4 Ma，属于皖南地区燕山期晚期岩浆作用的产物。风化壳可细分为残坡积层（A）、强半风化层（C1）、过渡层（C2）、弱风化层（C3）和基岩（D）五层。稀土总量在纵向剖面上呈“波浪式”分布，各层稀土分布型式表现出对原岩的继承性。风化壳稀土配分型式与基岩一致， 富集LREE，轻重稀土分馏明显(La/Yb)N=15.6），但总含量明显更高。基岩∑REE为338×10-6，半风化层∑REE最高达642×10-6，富集约两倍。风化壳物质由风化残余主矿物（石英、钾长石、斜长石、黑云母）、黏土矿物（高岭石、埃洛石、伊利石、三水铝石等）和副矿物（锆石、磷灰石、榍石等）等组成。黏土矿物以伊利石含量最高，指示风化壳发育不成熟。REE与埃洛石含量明显正相关，与其他黏土矿物关系不明显。（含）稀土矿物（尤其是榍石）对风化壳中稀土元素的贡献量超过百分之五十，其次为斜长石，是风化壳中REE的重要来源。