An Early Palaeozoic HP/HT granulite–garnet peridotite association in the south Altyn Tagh, NW China: P–T history and U-Pb geochronology     

J. X. ZHANG  C. G. MATTINSON  F. C. MENG  Y. S. WAN 《Journal of Metamorphic Geology》2005,23(7):491-510

High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas.  相似文献   

Phase petrology and P-T conditions of mafic blueschists from the Meliata unit, West Carpathians, Slovakia     

S. W. FARYAD 《Journal of Metamorphic Geology》1995,13(6):701-714

Abstract Blueschists occurring as layers in calcite marbles of the Meliata unit occur along the so-called Roznava tectonic line situated in the southern part of the Gemericum, Slovakia. Mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated. The most common minerals in the blueschists are blue amphibole, epidote and albite. Some Fe²⁺- and Al-rich rocks also contain garnet and chloritoid, respectively. Na-pyroxene with a maximum 50% jadeite component was also found. The blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples. Almandine- and spessartine-rich garnet has very low MgO content (<3 wt%). The Si content in phengite ranges between 3.3 and 3.5 pfu calculated on the basis of 11 oxygens. The zoning patterns of blue amphibole, garnet and chloritoid suggest their formation during a prograde stage of metamorphism. The P-T conditions of metamorphism are estimated to be about 380–460° C and 10–13 kbar. Pressures of 7.5–8.5 kbar and temperatures of 350–370° C were obtained for some actinolite- and aegirine-rich rocks. Apart from chlorite, other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite.  相似文献   

Contrasting mineral isograd sequences in metabasites of the Cape Smith Belt, northern Québec, Canada: three new bathograds for mafic rocks     

N. J. BÉGIN 《Journal of Metamorphic Geology》1992,10(5):685-704

Regional-scale mapping of index-mineral isograds in mafic units of the early Proterozoic Cape Smith Thrust Belt (northern Québec) has revealed contrasting pressure-temperature regimes associated with two distinct structural domains. In the southern domain, crustal thickening was accomplished by early, piggy-back thrust faults. Isograds cross-cut the thrusts, indicating that thermal-peak mineral growth outlasted deformation associated with early imbrication. Mineral zones are: (1) actinolite (Act) + albite (Alb); (2) hornblende (Hbl) + Act + Alb; (3) Hbl + Act + oligoclase (Oli); (4) Hbl + Oli; and (5) garnet (Grt) or clinopyroxene + Hbl + Oli-andesine. The oligoclase isograd occurs at higher grade than the hornblende isograd, a sequence typical of medium-pressure terranes (5–7 kbar). An Hbl-Alb bathograd. calibrated from mixed-volatile equilibria in the NCMASH-CO₂ model system, suggests minimum pressures of about 5.4 kbar. Metamorphism in the northern domain was a consequence of re-imbrication, by means of out-of-sequence thrust faults active during and after peak metamorphic conditions. Mineral growth was coeval with thrusting, as documented by the syn-kinematic garnet porphyroblasts. Compared to the southern domain, a different sequence of isograds in mafic rocks shows that the albite-oligoclase transition takes place in the garnet zone. Based on thermobarometry in garnet-hornblende rocks, the oligoclase isograd occurs in a temperature range of 525–600°C, typical of high-pressure terranes (7–10 kbar). Calibrated bathograds for the Hbl-Ms-Alb and Grt-Alb bathozonal assemblages, respectively in the KNCMASH-CO₂ and NCMASH model systems, indicate minimum pressures in the northern domain of 6.7 and 8.5 kbar. Higher-pressure series for this domain are explained by out-of-sequence thrusts exposing deeper crustal levels. For similar structural levels, only minor amounts of syn-deformational uplift (1–2 kbar and 50–75°C) are recorded in metabasites of this domain, compared to results in adjacent metapelites of the area (essentially isothermal uplift of 3–5 kbar). RESUME La bande du Cap Smith (nord du Québec) est une ceinture de chevauchement d'ǎge protérozoique inférieur, dominée par des roches mafiques. La cartographie d'isogrades à minéraux indicateurs dans les unités mafiques de la ceinture a révelé deux régimes contrastes de pression–température, chacun associéà des épisodes distincts d'épaississement crustal. Dans le domaine sud, des failles de chevauchement en-série sont responsables pour l'empilement tectonique. Les isogrades recoupent les failles, indiquant que l'apogée thermale a suivi l'emplacement initial des nappes de charriage. Les zones minérales sont: (1) actinote (Act) + albite (Alb); 2) hornblende (Hbl) + Act + Alb; (3) Hbl + Act + oligoclase (Oli); 4) Hbl + Oli; et (5) grenat (Grt) où clinopyroxene + Hbl + Oli-andésine. L'isograde d'oligoclase apparaǐt à plus haute température que l'isograde d'hornblende, une séquence typique des terrains de pressions moyennes. Un bathograde Hbl–Alb, calibréà partir d'équilibre mixte de volatiles dans le système NCMASH–CO₂, suggère des pressions minimales d'environ 5.4 kbar. Le métamorphisme dans le domaine nord de la ceinture a été le résultat d'une réimbrication, causé par des chevauchements hors-série actifs pendant et après l'apogée thermale. La croissance minérale fǔt synchrone au chevauchement, documentée par des porphyroblastes de grenat syn-cinénatique. Comparé au domaine sud, une différente séquence d'isogrades dans les métabasaltes montre que la transition albite–oligoclase se situé dans la zone à grenat. Par la thermobarométrie dans les roches à grenat–hornblende l'isograde d'oligoclase se situe dans un écart de température de 525–600°C, typique des terrains de hautes pressions (7–10 kbar). Des bathogrades calibrés pour les assemblages bathozonales Hbl–Ms–Alb et Grt–Alb, rcspectivement dans les systèmes KNCMASH–CO₂ et NCMASH, indique des pression minimales pour le domaine nord de 6.7 et 8.5 kbar. Une zonégraphie à plus haute pression pour ce domaine est expliquée par des chevauchements hors-série exposant des niveaux plus inférieurs de la croǔte imbriqué. Pour des niveaux structuraux similaries, des soulèvements syn-métamorphiques mineurs sont enregistrés dans les métabasaltes (1–2 kbar et 50–75°C), comparés aux métapélites adjacentes avec un soulèvement (essentially isothermal uplift of 3–5 kbar.  相似文献   

Complex Growth Textures in a Polymetamorphic Metabasite from the Kraubath Massif (Eastern Alps)   总被引：1，自引：0，他引：1  

FARYAD  SHAH WALI; HOINKES  GEORG 《Journal of Petrology》2004,45(7):1441-1451

Zoned garnet and amphibole occur in metabasites of the KraubathMassif, Eastern Alps, that contain relic magmatic clinopyroxene.The amphibole composition gradually changes from core (X_Mg =0·83) to rim (X_Mg = 0·6–0·7). A numberof compositional varieties of garnet occur in the metabasite.An older porphyroblastic garnet (Py_23–27, Alm_41–43,Grs_29–33) has two different compositional domains, onerelatively rich in Mg (Py_27–30) and the other rich inCa (Grs_35–38) with a low Mg (Py_20–25) content. Theyoungest variety, which forms rims on, or microveins in, theporphyroblastic garnet, has high Ca and low Mg (Grs_40–57,Py_2–7, Alm_46–51). The amphibole cores and garnetporphyroblasts are interpreted to represent minerals formedduring Variscan regional metamorphism under amphibolite-faciesconditions. Alpine metamorphism is represented by the most recentCa-rich and Mg-poor variety of garnet that coexists with theamphibole rims, epidote and chlorite. Fracturing in the porphyroblasticgarnet probably originated during retrogression of the Variscanamphibolite-facies assemblages. Textural relations suggest thatthe garnet in the microveins formed by dehydration of hydrousphases during an Alpine metamorphic overprint that reached P–Tconditions of 550–583°C at 1·0 GPa. KEY WORDS: microveins; garnet; metabasites; Kraubath Massif; Eastern Alps  相似文献   

东秦岭松树沟高压变质基性岩石及其退变质作用的PTt演化轨迹   总被引：10，自引：2，他引：10  

刘良  周鼎武  董云鹏  张宏发  刘养杰  张泽军 《岩石学报》1995,11(2)

在北秦岭商南松树沟出露的高压变质基性岩石，包括高压基性麻粒岩和石榴单斜辉石岩，遭受了从中压麻粒岩相、高角闪岩相到绿帘角闪岩相的连续的退变质作用的改造。这些岩石形成的温度为８２６℃～８８７℃，压力约为１．４０～１．５８ＧＰａ，其不同退变质阶段的温压条件依次分别是７６５℃～８２５℃和约１．０３～１．１４ＧＰａ、６５０℃～７５０℃和约０．９ＧＰａ以及４００℃～５００℃和约０．５～０．６ＧＰａ。它们共同构成一个早期近等温降压（ＩＴＤ）和晚期降温降压的顺时针ＰＴｔ演化轨迹。结合高压岩石９８３±１４０Ｍａ的退变质年龄和松树沟残存的蛇绿岩片考虑，这些高压岩石的形成和抬升与秦岭造山带中、晚元古宙时期洋壳的俯冲、消减和陆块碰撞伴随的快速构造上升作用有关。  相似文献   

Petrology,geochemistry and zircon U–Pb dating of Band-e-Hezarchah metabasites (NE Iran): An evidence for back-arc magmatism along the northern active margin of Gondwana     

《Chemie der Erde / Geochemistry》2015,75(2):207-218

The Band-e-Hezarchah granitoids (BHG) is located in the northern margin of the central Iran, where the very old continental crust of Iran is found. The BHG mainly include granodiorite, granite and leucogranite. Small meta-gabbroic stocks and dykes are associated with BHG. U–Pb zircon dating of the BHG granites and metabasites yield ²³⁸U/²⁰⁶Pb crystallization ages of ca. 553.6 and 533.5 Ma respectively (Ediacaran–early Cambrian). The metabasites have calc-alkaline signature and their magmas seem to have originated from a mantle wedge above a subduction zone. These rocks are thought to be formed in a continental back-arc setting, related to the oblique subduction of Proto-Tethys oceanic lithosphere beneath the northern margin of Gondwanan supercontinent during Ediacaran–Cambrian time. The initial ⁸⁷Sr/⁸⁶Sr ratios and ɛNd (t) values for metabasites are change from 0.705 to 0.706 and −3.5 to −3.6 respectively. Sr–Nd isotope composition of metabasites indicates that these rocks were derived from a subcontinental lithospheric mantle source. The BHG and associated metabasites are coeval with other similar aged metagranites and gneisses from Iranian basements exposed in central Iran, Sanandaj-Sirjan and Alborz zones. These rocks were formed due to continental arc magmatism of Neoproterozoic–early Cambrian, bordering the northern active margin of Gondwana.  相似文献   

新疆西南天山哈布腾苏一带高压-超高压变质基性岩的地球化学特征及其构造意义   总被引：1，自引：1，他引：0  

施建荣  杨红  刘福来  孟恩  刘平华  王舫  蔡佳 《岩石学报》2013,29(6):2251-2264

新疆西南天山哈布腾苏地区的高压-超高压变质基性岩(榴辉岩和蓝片岩)主要以形态各异的岩块或构造透镜体形式产于其围岩变质沉积岩石榴多硅白云母片岩中.本文对这些变质基性岩进行了详细的岩石学、矿物学和地球化学研究,并对其原岩性质及区域构造演化进行了讨论.地球化学分析结果显示其原岩成分主要为拉斑玄武岩和少量碱性玄武岩.榴辉岩原岩可进一步划分为N-MORB(正常洋中脊玄武岩)、E-MORB(富集洋中脊玄武岩)和OIB(洋岛玄武岩)三种类型,而蓝片岩为OIB型.第一种榴辉岩具低的TiO2(1.00％ ～ 1.08％)、P2O5 (0.09％ ～0.15％)和∑REE(35.98×10-6 ～ 43.51×10-6)含量以及低的LREE/HREE(1.35 ～ 1.43)和(La/Yb)N(0.63 ～0.71)比值等特征,具有N-MORB属性,暗示其原岩可能来源于亏损的地幔源区,形成于大洋环境;第二种榴辉岩具有较高的Ti(TiO2=1.35％ ～2.25％)、P(P2O5 =0.05％ ～0.20％)和∑REE(60.10×10-6～ 77.35×10-6)含量和较高的LREE/HREE(1.25 ～2.43)、(La/Yb)N (0.52～1.52)比值,具有E-MORB特征;第三种榴辉岩TiO2含量为1.16％ ～2.86％,Zr/Y=5.32 ～ 7.78,∑REE=79.12×10-6 ～ 192.1×10-6,LREE/HREE =4.15 ～6.54,(La/Yb)N =3.77 ～9.44,其轻重稀土元素分异明显,显示OIB属性,暗示其原岩可能来源于富集地幔源区.而OIB型蓝片岩则富集TiO2(1.39％～2.86％)和∑REE(88.18×10-6～227.8×10-6)和具有相对高的(La/Yb)N(5.03～9.84)和Zr/Y(4.93 ～9.55)比值,这些特征与OIB型榴辉岩类似,暗示二者可能具有共同的岩浆源区.新疆西南天山哈布腾苏地区具MORB和OIB属性岩石组合的出现揭示该区早古生代曾发育有洋盆,其演化过程中可能受到结晶分异作用以及异源岩浆混合的影响,导致该区的变质基性岩元素地球化学特征具有MORB和OIB的亲缘性,暗示其原岩是在大洋中脊和临近洋岛共同作用背景下形成的.该区榴辉岩和蓝片岩的形成可能与先存洋壳残片经历高压-超高压变质作用及后期构造折返有关.  相似文献   

Petrology and tectonic significance of metabasite slivers in the Lesser and Higher Himalayan domains of Sikkim,India     

K. FAAK  S. CHAKRABORTY  S. DASGUPTA 《Journal of Metamorphic Geology》2012,30(6):599-622

The metamorphic history of the Himalayas has been constrained mostly through studies of the ubiquitous metapelitic rocks. Non‐eclogitic metabasite rock lenses that occur intercalated with the metapelites have received little attention and it is not clear whether they share a common metamorphic history. This study reports the results of a petrological study of the metabasite lenses (dm³–m³) from the Lesser Himalayan (LH) and the Higher Himalayan (HH) domains in Sikkim. These have similar bulk chemical compositions and chemical affinities (sub‐alkaline tholeiitic basalts), with plagioclase and amphibole as the dominant mineralogical constituents. Garnet and clinopyroxene occur in some samples depending on small variations in bulk chemistry; and orthopyroxene is developed as a retrograde phase in some rocks. Minor phases are ilmenite, chlorite, titanite and rutile. The rocks were metamorphosed at similar conditions (～9–12 kbar, 800 °C). Minor differences in bulk chemical composition lead to different phase assemblages and mineral chemistry in adjacent metabasite lenses, a feature that is used to demonstrate that metamorphic conditions (peak P–T as well as retrograde P–T path) can be reliably retrieved through a combination of pseudosection analysis and kinetically constrained individual thermobarometry. The peak P–T conditions of the metabasites from this region are independent of the present geographic or tectonic (i.e. within the LH or HH) location of the samples and they differ from the conditions at which the regional metapelites (i.e. metapelites not immediately adjacent to the metabasite lenses) were metamorphosed. Metapelites that are immediately adjacent to the metabasite lenses differ in their appearance, phase assemblage and recorded P–T history from those of the regional metapelites, either because they were emplaced as slivers along with the metabasites, or because they were modified when they came in contact with the metabasites. The retrograde P–T paths of the LH and HH metabasites are different: the HH samples underwent steep decompression whereas the LH followed a more gentle exhumation path. The P–T conditions of peak metamorphism (9–12 kbar, 800 °C) are commensurate with a thermal perturbation at the base of a crust of average thickness and may be the signature of a widespread (samples found across different regions in the Himalaya) and long‐lasting (e.g. homogeneous garnet compositions) crustal underplating event that occurred during the early stages (?subduction) of the Himalayan orogeny, or earlier if the metamorphism was pre‐Himalayan.  相似文献   

Subdivision of the Sanbagawa pumpellyite–actinolite facies region in central Shikoku, southwest Japan     

Masumi  Sakaguchi  Hideo  Ishizuka 《Island Arc》2008,17(3):305-321

Abstract   The mineral assemblages of the pumpellyite–actinolite facies such as pumpellyite + actinolite + epidote + chlorite or actinolite + epidote + hematite + chlorite occur in the Sanbagawa low-grade metamorphic region, central Shikoku, southwest Japan. Chemical compositions of these minerals from the eight newly studied areas were analyzed in order to evaluate the areal extent and thermal structure of the region. In the buffered assemblage of pumpellyite + actinolite + epidote + chlorite, the Fe³⁺/(Fe³⁺ + Al) values of epidote decrease slightly with decreasing Fe²⁺/(Fe²⁺ + Mg) values for chlorite. The changes in these values show a general correlation with temperature. The presence of this relationship implies that the Fe³⁺/(Fe³⁺ + Al) values of epidote can be used to divide the Sanbagawa low-grade metamorphic region into low-, medium- and high-grade subzones. The areal distribution of these subzones indicates that: (i) the temperature seems to decrease in the same sense as envisaged by the zonal mapping of the higher-grade pelitic schists; and (ii) there is no significant gap of metamorphic conditions through the boundary between the two structural units (Besshi and Oboke units). It follows that the Sanbagawa low-grade metamorphic region decreases in temperature going up the structural section, and tectonic discontinuities have not affected the thermal structure.  相似文献   

浙江龙泉地区变质基性岩年代学、地球化学特征及构造意义   总被引：1，自引：0，他引：1  

刘远栋  刘风龙  张建芳  李翔  徐磊  温旭斌  倪伟伟 《地质学报》2021,95(2):413-426

浙西南龙泉地区集中出露一套中深变质岩系,是华夏地块的重要组成部分,对其形成时代和成岩构造属性等问题历来存在争议。本文通过对龙泉变质岩中变质基性岩的地球化学研究,认为这些岩石的原岩成岩环境至少有两类:Ⅰ类岩石稀土配分曲线、微量元素蛛网图特征均显示与IAB的相似性,微量元素比值和图解判别均指示这套岩石形成于与俯冲有关的岛弧构造环境;Ⅱ类岩石稀土配分曲线和微量元素蛛网图均与OIB有相似性,微量元素比值和图解判别也指示这套岩石形成于与俯冲无关的OIB构造环境。多种不同构造属性的岩石共生,佐证了原龙泉岩群变质岩系为一形成于早古生代的俯冲增生杂岩。变质基性岩锆石U-Pb测年获得其峰期变质时间为404~402Ma。龙泉俯冲增生杂岩的厘定对重新认识丽水-余姚断裂带构造属性、区域构造格架的建立以及进一步分析华夏造山系物质组成与结构提供了新资料。  相似文献