Mineral Chemistry and Thermobarometry of the Upper Eocene Volcanic Rocks in NE Tafresh, Iran

Petrography and chemistry of minerals show that rocks of Upper Eocene in northeast of Tafresh are composed mostly of andesitic basalt, basaltic andesite and andesite volcanic rocks. Mineralogically these rocks are composed of phenocrystals of olivine, clinopyroxene and plagioclase and main texture of them is porphyry with cryptocrystalline or microcrystalline matrix. In addition, aphyric and pitted textures (amygdala) are also observed. According to the results of EPMA, phenocrystals of plagioclase in mentioned rocks include a range of anorthite to albite minerals. Alkali feldspars also contain a range of sodic to potassic minerals. Pyroxene crystals include hedenbergite, augite and hypersthene. Olivine minerals are often of the ferrohornblendite type. Based on thermobarometry it is estimated that to form clinopyroxene crystals of basaltic andesite rocks, temperature between 750°C to 1000°C is needed. Andesitic basalt rocks at higher temperature (1100°C) and andesite rocks at lower temperature (below 750°C) are formed. According to the distribution of aluminum in clinopyroxenes, these minerals at pressures less than 5 kbar and water content between 5% to 10% are crystallized. The mineral composition indicates that these rocks are formed in a tensional environment.

Geothermobarometry of Askaoun Pluton in Ouzellarh-Sirwa Promontory (Central Anti-Atlas;Morocco)

Rocks of the late Neoproterozoic Askaoun pluton (558 ± 2 Ma) located in the Ouzellarh-Sirwa promontory (Anti-Atlas) include magmatic microgranular enclaves (MMEs) ranging from rounded to ovoid in shape, dark and fine grained and generally 5 to 10 cm in size, some reaching a size of 50 cm. They are composed of microdiorite, quartz microdiorite and micromonzodiorite, whereas the felsic host rocks comprise mainly quartz-diorite and amphibole-biotite granodiorite based on mineralogical compositions. The mineral assemblage is similar to those described in their hosting granitoids but with different proportions. In this study composition of minerals is used to describe the nature of the magma and estimate the pressure, temperature and oxygen fugacity at which Askaoun pluton is emplaced. Based on chemistry of biotite Askaoun pluton formed from calc-alkaline magma. Compositions of plagioclase (An5 - An29);hornblende (Mg ≠= 0.59 - 0.65) and biotite (Mg ≠= 0.49 - 0.55) of MMEs are slightly distinct or similar to those of host rocks (An7-40;hbl Mg ≠= 0.64 - 0.69;Bi Mg ≠= 0.49 - 0.50) which suggest partial to complete equilibration during mafic-felsic magma interaction. The coexisting hornblende and plagioclase (hornblende-plagioclase thermometry), Al content in hornblende (aluminum-in-hornblende barometry) and the assemblage titanite-magnetite-quartz were used to constrain the P, T and fO2 during the crystallization of the parent magmas. The Askaoun pluton was emplaced at temperature ca. 504°C - 633°C and at pressure ca. 0.9 - 4.66 ± 0.6 Kbars (average depth = 6.5 km) from a highly oxidized magma (log fO2 = ?24.8 to ?19.2).

Geochemistry, Mineral Chemistry and Thermobarometry of Boneh-Shorou Amphibolites in Gelmandeh Massive (Saghand-Central Iran)

The Gelmandeh Massive is located in the Central part of Iranian Microcontinent. Amphibolitic rocks occupy the main portion of Gelmandeh complex, which consists of hornblendite, garnet amphibolites, and gneiss. Geochemical investigations indicate that the ratio of Eu/Eu* separates samples into two categories: first category with ratio of Eu/Eu* > 1 and the second with ratio of Eu/Eu* < 1, which the former indicates an enriched mantle origin while the latter points to crustal contamination. Generally, amphibolites can be differentiated into two separate series. First series is characterized with calc-alkaline to alkaline composition with enriched mantle origin, and the second one has calc-alkaline to tholeiitic nature and it is of continental type that underwent crustal contamination within subduction zone. The microprobe electron analysis shows that the amphiboles were a member of the calcic group and hastingsite-tschermakite series. Geo-barometric studied and mineral paragenesis show that the metamorphism is of Barovian type with PT condition upto 7 K bar and 580°C.

视剖面图温压计研究进展评述

视剖面图温压计是目前定量估算变质温度和压力的主要方法之一。其理论基础为热力学平衡原理与质量守恒定律。利用内恰的热力学数据库和相应的成分-活度模型对特定成分体系进行视剖面图计算,可以正演模拟给定温度和压力条件下的矿物组合、矿物丰度及其成分,与实测岩石对比,可以准确获取岩石的形成温度和压力。在应用视剖面图温压计时,应选择受全岩成分影响小、且受退变质作用影响微弱的矿物成分;对于有成分变化的体系,应结合多种方法恢复有效全岩成分。文中通过视剖面图对KFMASH(K2O-Fe O-Mg O-Al2O3-Si O2-H2O)体系下不同矿物组合内多硅白云母硅含量及其等值线斜率的研究,认为多硅白云母硅含量及其等值线斜率主要由不同矿物组合内多硅白云母参与的契尔马克替换及纯转换端元反应中的主导反应决定,进一步深化了对变质过程中控制矿物成分变化的内部缓冲反应机制的理解,从而也可为选择和应用矿物温压计提供指导。因此,视剖面图方法是目前研究变质岩石形成条件及变质作用精细过程的最佳方法之一。

铜陵地区中酸性侵入岩及其包体的成因和矿物温压计的应用

铜陵地区中酸性侵入岩可划分为橄榄安粗岩系列和高钾钙碱性系列。前者岩石组合为辉石二长闪长岩+二长岩+石英二长岩,后者为辉长闪长岩+石英二长闪长岩+花岗闪长岩。两个系列岩石的组成矿物种类相似,但在不同岩石中的含量不同,主要造岩矿物为斜长石、辉石、角闪石、黑云母、钾长石、石英。橄榄安粗岩系列岩石中含有大量的深源包体,包括辉石堆积岩包体、角闪石堆积岩包体和角闪石辉长质堆积岩包体,主要矿物为辉石、角闪石,其次为尖晶石、斜长石、磷灰石、金云母;高钾钙碱性系列岩石中含大量的微粒闪长质包体、镁铁质石英二长闪长质包体和富云母包体,主要矿物为斜长石、角闪石、黑云母。在考虑温压计使用条件的前提下,选择合适的温压计计算了两个系列侵入岩及包体形成的温压条件。结果表明,橄榄安粗岩系列侵入岩侵位深度(4～6km)略小于高钾钙碱性系列侵入岩(6～7km),各种堆积包体形成于45～65km的深位岩浆房,微粒闪长质包体形成于12～15km的浅位岩浆房,镁铁质石英二长闪长质包体是早期侵入岩的边缘相,而富云母包体可能为地壳部分熔融的残余相。

东天山觉罗塔格构造带雅满苏组火山岩的矿物学研究

东天山觉罗塔格构造带雅满苏组的构造背景一直存有较大争议。斑晶成分和结构的研究可以对岩浆系列、构造背景和岩浆演化的物理化学条件作出制约。本文报道了雅满苏组火山岩中斜长石和辉石斑晶的研究成果。辉石属于普通辉石-顽透辉石系列，部分样品的辉石颗粒与寄主岩达到了岩浆平衡。辉石判别图解表明寄主岩浆属于亚碱性系列，形成于俯冲带环境，与微量元素判别结果相一致。根据辉石-熔体温压计，得到辉石结晶时的温度在1150℃左右，压力相当于5~10 km的地壳深度。雅满苏组火山岩中主体的斜长石属于拉长石，JX-24-1斜长石为中长石，成分和结构特征都表明该颗粒为捕虏晶。辉石和斜长石捕虏晶共同揭示了雅满苏组火山岩在地壳浅部发生了结晶分异和围岩混染的岩浆过程。

点苍山-哀牢山杂岩带中北段嘎洒地区变沉积岩的成因矿物学与变质演化特征

点苍山-哀牢山变质杂岩带中北段嘎洒地区出露了多种典型的变沉积岩，其中夕线石榴黑云二长片麻岩和二云母片岩保存多期/多阶段矿物相转变特征，本文通过岩相学和矿物化学的综合分析，并结合传统矿物对温压计的估算结果，限定上述典型变沉积岩峰期角闪-麻粒岩相（M1）阶段、近等温减压-高温剪切变形阶段（M2）和晚期退变质（M3）阶段的矿物组合及变质温压条件。峰期角闪-麻粒岩相（M1）阶段的矿物组合为：石榴石（Grt）＋板柱状夕线石（Sil1）＋黑云母（Bt1）＋钾长石（Kfs）＋斜长石（Pl）＋石英（Qtz）＋钛铁矿（Ilm），变质温度压力条件为t=690～750℃，p=690～810 MPa；近等温减压-高温剪切变形阶段（M2）阶段，稳定矿物组合为：Grt＋Sil2＋Bt2＋Kfs＋Pl＋Qtz＋Ilm，黑云母在强烈走滑剪切作用下发生脱水熔融反应：2 Bt→Sil＋6（Mg，Fe）O＋K2O＋5 Qtz＋2 H2O，石榴石、黑云母和夕线石等受到剪切变形影响而发生强烈定向，形成的温度压力条件为t=650～720℃，p=450～630 MPa；晚期退变质阶段（M3）的稳定矿物组合为：Qtz＋Bt＋Ms＋Pl，退变的温度压力条件为t=580～640℃，p=400～500 MPa。其变质演化p-T轨迹样式具有近等温减压的顺时针型式，表明点苍山-哀牢山变质杂岩带曾经历了一次明显的俯冲-碰撞造山事件，峰期变质可达到角闪-麻粒岩相；在碰撞后的构造折返过程中，上述变质岩石发生强烈的高温剪切变形作用，并伴随着黑云母等含水矿物的脱水熔融。

安徽铜陵地区虎山岩体矿物成分特征及其锆石U-Pb年代学

铜陵地区虎山岩体主要由石英二长闪长岩和花岗闪长斑岩组成,属高钾钙碱性系列中酸性侵入岩。本文对虎山岩体进行了LA-ICP-MS锆石U-Pb定年,结果表明,虎山石英二长闪长岩和花岗闪长斑岩侵入时代分别为（143.2±1.2）Ma和（145.3±1.2）Ma,属晚侏罗—早白垩世,与中国东部J-K岩浆事件相呼应,并且,虎山岩体存在两次岩浆侵入活动,早期形成的花岗闪长斑岩的年龄为（145.3±1.2）Ma,与铜陵矿集区瑶山花岗闪长斑岩的年龄（146.0±0.9）Ma在误差范围内一致,晚期形成的石英二长闪长岩为（143.2±1.2）Ma,与区内的铜官山和天鹅抱蛋山岩体的年龄（142±1）Ma和（141.7±1）Ma一致。岩石地球化学及矿物温压计计算结果表明,岩浆结晶的温度为768~838°C、压力为3.44×108~5.42×108Pa,虎山岩体属幔源岩浆和壳源岩浆混合成因,且混合作用发生在11~18 km的浅位岩浆房。

基性麻粒岩造岩矿物微量元素再分配特征及其对变质历史的指示作用：以胶北地体高压麻粒岩为例

高级变质岩的变质历史是反演地壳构造-热事件的重要依据,然而高温扩散和重结晶作用能够改造造岩矿物中的主量元素分布,这对峰期变质温压条件的反演产生很不利的影响。相对于主量元素,微量元素,尤其是离子半径较大的REE,由于其在晶格中的扩散速率远小于主量元素,在高级叠加变质过程有可能记录前期变质作用。本文以胶北地体的高压基性麻粒岩为研究对象,通过详细的岩相学和矿物化学分析,初步解析了变质重结晶过程中的矿物微量元素再分配特征及其对变质作用的指示意义。岩相学上的证据表明这些样品经历了麻粒岩相变质和后期重结晶作用。单矿物的原位化学成分分析,峰期矿物石榴石、单斜辉石的主量元素Mg、Fe、Ca等二价阳离子分布均一,但部分稀土元素及微量元素则表现出钟形剖面环带分布,暗示主量元素遭受到成份扩散及重结晶所致的元素再分配,微量元素可记录峰期历史。结合主、微量元素温压计,我们分别估算了胶东基性高压麻粒岩的峰期（〉828℃、〉1.27GPa）和中压麻粒岩相退变质温压条件（810~840℃、0.6~1.0GPa）,并推测其后期经历过角闪岩相退变质叠加。结合前人的年代学工作,我们认为该基性麻粒岩经历了近等温快速减压的变质历史。

青海共和盆地东北部干热岩岩浆侵位结晶条件及深部结构初探

秦岭、祁连、昆仑造山带结合处的共和盆地记录了复杂的构造、岩浆、变质和沉积作用事件,是研究青藏高原北部构造-岩浆演化的关键地区。近年来,共和盆地花岗岩类高温干热岩的发现使得该区成为新型地热资源的战略基地。然而,有关干热岩原岩的组成、侵位时代、结晶温压及其时空分布等特征仍不清楚,制约着干热岩地热能勘查开采及该区构造-热演化历史的认识。本文以共和盆地东北部恰卜恰地区3口干热岩钻井岩芯和野外露头样品为基础,结合区域地质调查资料,开展了系统的岩石学、锆石U-Pb年代学和矿物温压计研究。研究发现,该区干热岩主要由花岗闪长岩、英云闪长岩、二长花岗岩和正长花岗岩等花岗岩类组成,偶见闪长岩包体。电子探针分析显示,样品中多数角闪石为铁角闪石。角闪石-斜长石矿物对温压计研究表明,岩体的结晶压力属于中低压(1.91~3.52kbar),温度为中低温(681~693℃),其岩浆结晶深度约在7.2~13.2km。锆石Ti饱和温度计分析表明,该区岩浆结晶温度主要为643~804℃,而恰卜恰北部的沟后杂岩体相较干热岩钻井岩芯钻揭的杂岩体具有更高的结晶温度。锆石U-Pb年代学分析表明,共和盆地东北部恰卜恰地区干热岩原岩形成时间主要为243~236Ma和225~210Ma两个时期,存在不同时期、不同源区的侵入,与印支期青藏高原北部古特提斯洋盆的俯冲消减相关。之后,共和干热岩经历了至少4.2km的隆升剥露。结合前人多种地球物理研究资料,构建了该区深部四层结构构造剖面,干热岩热源可能与地幔上涌、中下地壳存在局部熔融体有关,且受多条隐伏断裂的影响,盆地基底下的干热岩地热藏具有分区性。

Comparison of mantle lithosphere beneath early Triassic kimberlite fields in Siberian craton reconstructed from deep-seated xenocrysts

Mantle xenocrysts from early Triassic kimberlite pipes from Kharamai, Ary-Mastakh and Kuranakh fields in the Anabar shield of Siberia revealing similar compositional trends were studied to estimate the superplume influence on the subcratonic lithosphere mantle （SCLM）. Pressure-temperature （PT） reconstructions using monomineral thermobarometry for 5 phases show division of the SCLM beneath the Kharamai field into 6 units： pyroxenitic Fe-rich （1-2 GPa） and Mg-rich （2-3 GPa） layers; middle with two levels of Gar-Sp pyroxenites at - 3 and 4-5 GPa; Gar-dunite-harzburgites - 4.5-6.5 GPa subjected to llm-Px vein metasomatism; and a Mg-rich dunite lower part. In the Anabar shield （Ary-Mastakh, Dyuken and Kuranakh fields） mantle lithosphere is composed of three large units divided into two parts： upper part with amphiboles and phlogopite; two levels of pyroxenites and eclogites at 3 and 4 GPa, and a lower part composed of refertilized dunites. Diagrams showing P-Fe#Gar clusters for garnets and omphacites illustrate the differences between SCLM of these localities. Differences of Triassic SCLM from Devonian SCLM are in simple layering; abundance of Na-Cr-amphiboles and metasomatism in the upper SCLM part, thick pyroxenite-eclogite layer and lower part depletion, heated from SCLM base to 5.0 GPa. Kharamai mantle clinopyroxenes represent three geochemical types： （1） harzburgitic with inclined linear REE, HFSE troughs and elevated Th, U; （2） lherzolitic or pyroxenitic with round TRE patterns and decreasing incompatible elements; （3） eclogitic with Eu troughs, Pb peak and high LILE content. Calculated parental melts for garnets with humped REE patterns suggest dissolution of former Cpx and depression means Cpx and garnets extraction. Clinopyroxenes from Ary-Mastakh fields show less in- dined REE patterns with HMREE troughs and an increase of incompatible elements. Clinopyroxenes from Kuranakh field show flatter spoon-like REE patterns and peaks in Ba, U, Pb and St, similar to those in ophiolitic harzburgites. The PT diagrams for the mantle sections show high temperature gradients in the uppermost SCLM accompanied by an increase of P-Fe#OI upward and slightly reduced thickness of the mantle keel of the Siberian craton, resulting from the influence of the Permian-Triassic superplume, but with no signs of delamination.

How High Are the P-T Conditions for Paleoproterozoic Metamorphism of the Huangtuling Felsic Granulite, North Dabieshan, Central China?

This paper is supported by the Special Doctoral Grant of the Ministry of Education of China （No. 98049114） and the National Natural Science Foundation of China （No. 49972023）.

Magma Mixing as a Trigger of Unzen's Volcano Eruptions

Recent eruption in 1991-1995 years of Unzen Volcano(Kyushu Island,Japan)has caught attention of many volcanologists because of dramatical consequences of the previos eruption in 1792,when more than 15000 people were perished.So it is important to study this volcano and try to predict future eruptions and their possible damage.It is proved now,that magma hybridization processes

Tectonochemistry and p-t Conditions of Ramgarh and Almora Gneisses from Askot Klippe,Kumaun Lesser Himalaya

The chemical and petrological correlation of metamorphic nappes and klippes overlying the Proterozoic sedimentary units in the Kumaun Himalaya is still debated. The Ramgarh and Almora gneisses, not previously distinguished in the Askot Klippe, show distinct field, petrological and chemical signatures markedly similar to the tectonostratigraphic disposition of the Almora Nappe. A negative Eu anomaly in the Ramgarh granitic gneisses indicates lesser plagioclase fractionation while the Eu anomaly in the Almora pelitic gneisses is likely to have been controlled by feldspar crystallization in restites. During the anatexis at > 776°C temperature and >6.6 kbar pressure, the melt moved slightly away to its crystallization sites. The Rb/Sr ratio ?0.54 and Nb ?10 ppm is consistent with the granodioritic composition. The negative Sr anomaly in the underlying Ramgarh granitic gneisses indicates a distinct mantle derived source/plagioclase fractionation with a notable correspondence to other late orogenic granites, particularly the basement Ulleri gneisses from the Nepal Himalaya. Ramgarh gneisses plot in the late-and post-COLG field. The Askot ensemble is likely to be the tectonometamorphically reworked basement, viz. the Ramgarh Group along with its metapelitic cover o f the Almora Group, together comprising southward thrust remnants of the leading edge of the Indian Plate that collided with Tibet during the Tertiary Himalayan orogeny.

Alakit and Daldyn kimberlite fields,Siberia,Russia:Two types of mantle sub-terranes beneath central Yakutia?

Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle.Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields.In the Alakit field.Cr-diopsides are alkaline,and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions,while in the Daldyn field harzburgitic pyropes are frequent.The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions,while in the Daldyn field they show varying compositions and often continuous Pressure-Temperature（P-T） ranges with increasing Fe~# with decreasing pressures.In Alakit,Crpargasites to richterites were found in all pipes,while in Daldyn,pargasites are rare Dalnyaya and Zarnitsa pipes.Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements（LREE）and stronger REE-slopes,and enrichment in light Rare Earth Elements（LREE）,sometimes Th-U,and small troughs in Nb-Ta-Zr.In the Daldyn field,the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances,while those from sheared and refertilized peridotites have smooth patterns.Garnets from Alakit show HREE minima,but those from Daldyn often have a trough at Yand high U and Pb.PTX/O2 diagrams from both regions show similarities,suggesting similar layering and structures.The degree of metasomatism is often higher for pipes which show dispersion in P-Fe~# trends for garnets.In the mantle beneath Udachnaya and Aykhal,pipes show 6-7 linear arrays of P-Fe~# in the lower part of the mantle section at 7.5-3.0 GPa,probably reflecting primary subduction horizons.Beneath the Sytykanskaya pipe,there are several horizons with opposite inclinations which reflect metasomatic processes.The high dispersion of the P—Fe~# trend indicating widespread metasomatism is associated with decreased diamond grades.Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel.Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins.Incorporation of island arc peridotites from an eroded arc is a possible scenario.

Mantle Transects in Africa According to Data of Mantle Xenocrysts and Diamond Inclusions

We designed the mantle transects using the PTXFO2 diagrams(Ashchepkov et al.,2010,2013,2017)constructed(Figs.1 a–c)for mantle columns beneath kimberlite and sections of the lithospheric mantle(SCLM)under the Kaapvaal and the Congo cratons.The set of the pipes is in(Zinchenko et al.,2020,2021).

Thermal State and Structure of the Lithosphere beneath Eastern China: A Synthesis on Basalt-Borne Xenoliths

Application of reliable thermobarometer on garnet-bearing mantle xenoliths and granulite xenoliths entrained by Cenozoic basalts in eastern China reveals two main types of geotherm. The first type, as exampled by Hannuoba （汉若坝）, Mingxi （明溪） and probably Northeast China, is characterized by constant slope of data in the P-T space. The second type, as exampled by the high geotherms of Niishan （女山） and probably Xinchang （新昌）, is characterized by variable slopes, with the samples with pressure 〈2 MPa defining a slow slope, whereas the samples with pressure 〉2 MPa define a virtually vertical slope. The different slopes in the second type of geotherm may correspond to different heat transfer mechanisms, with conductive transfer for the shallow upper mantle and advective transfer for the deep mantle. This observed transition in thermal transfer mechanism is consistent with theoretical modeling. The two types of geotherm are not mutually exclusive, because the second type may characterize the thermal state of whole lithospheric section including both mechanical boundary layer （MBL） and thermal boundary layer （TBL）, while the first type may only depict the MBL. The variable geotherms for different regions are indicative of a heterogeneous lithospheric structure in eastern China. （a） Eastern North China craton （NCC） is characterized by a second-type geotherm, corresponding to a thin lithosphere （-70 km）. Comparison of the equilibrium temperatures of spinel peridotites with this geotherm constrains the depth to Moho in eastern North China craton to be 30 kin. In contrast, western NCC （Hannuoba： the first-type geotherm） possesses a relatively low thermal gradient, indicative of a thick lithosphere （〉90-100 km） and a thick crust-mantle transition zone. The dramatic change in crustal and mantle structure across the DTGL （Daxing＇anling （大兴安岭）- Talhangshan （太行山） gravity lineament） is consistent with recent seismic studies. （b） There is a decrease in thermal gradient and in lithospheric thickness from the coast （Xinchang： the second-type geotherm） to the inland （Mingxi： the first-type geotherm） in South China （from -80 km to 〉90 kin）, which is collaborated with westward variation in basalt geochemistry. （c） The weak convex-upward pattern of the geo- therm in Qilin （麒麟） and Leizhou （雷州） Peninsula is peculiar, probably reflecting a transitional feature between conductive and advective heat transfer. It may result from impregnation of mantle plume on the base of the lithosphere. These new results not only provide a basic framework for the ongoing 4-D lithosphere mapping project in eastern China, but also yield important implications for deep processes that operated over the past.

Is the Songshugou Complex,Qinling Belt,China,an Eclogite Facies Neoproterozoic Ophiolite?

Ophiolites play a key role in understanding subduction-accretion-collision processes. Herein, we discuss origin and metamorphic evolution of an enigmatic, Neoproterozoic ophiolite candidate—the mafic-ultramafic Songshugou Complex, Qinling belt, China—summarizing published thermobarometr);U/Pb geochronology, and geochemistry and presenting new phase equilibrium modeling. Garnet, rarely preserved in amphibolites of the Songshugou Complex, has prograde zoning and low-pvrope cores |Alm54-71(Grs+And)25-30PrP1-6Sps5-12. It formed at quartz eclogite facies conditions of 1.93-2.54 GPa, 462-542℃. During exhumation, garnet first was mantled by plagioclase-rich coronas at about 0.7-1.2 GPa, 660-710℃. During an isothermal uplift to 0.5-0.8 GPa, these coronas evolved widely into n-shaped aggregates and eventually into whitish ribbons oriented with a steeply southwest dipping mineral stretching lineation. The exhumation into middle-upper crustal levels proceeded till the Late Devonian. The oceanic protoliths of the eclogites were emplaced into continental crust in the Neoproterozoic and dragged into a subduction zone in North Qinling in the Cambrian. The ultramafic rocks of the Songshugou Complex were not subducted with the mafic rocks in a coherent block given the absence of garnet but ubiquitous occurrence of spinel implies a P maximum of ?1.7 GPa. Rather, mafic and ultramafic rocks belonged to downgoing and overriding plate, respectively. They were juxtaposed at 0.8-1.7 GPa at Early Ordovician time.

Temperature and pressure condition of garnet Iherzolite and websterite from west Qinling,China

The mineral thermobarometry proposed in literature is used to calculate the equilibrium temperature and pressure of garnet Iherzolite and websterite xenoliths within the Cenozoic kama-fugite from west Qinling, Gansu Province, China. The results show that the equilibrium temperature and pressure of garnet Iherzolites and websterite and 1127-1266℃, 2.9-3.6 Gpa and 1169 -1248℃, 2.8-3.2 Gpa respectively. The equilibrium peressures reach or exceed the equilibrium peressure of spinel Iherzolites (2.0-3.0 GPa), and fall into the stability range of garnet peridotite. The equilibrium temperature of the xenoliths reach or exceed the ocean geotherm, identical with the melting temperature of kamafugite magma determined by experiments under the conditions of post-orogenic lithosphere extension. So the thermal state of Cenozoic mantle of the west Qinling may be fit to generate the kamafugite magmatism. The research on petrology-mineralogy and geobarothermometry of the xenoliths shows that both garnet Iherzolite and websterite are mantle components of the west Qinling, and may be considered as source rocks of the Cenozoic kamafugite magma.