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1.
Partial melting experiments on Nigerian charnockitic monzonite associated with migmatised gneisses provide consanguineous liquids from which the metaluminous and peraluminous younger granites may be derived. After 40–60% melting at 750–800° C the liquids are metaluminous with hastingsite-fayalite granite affinities. On cooling towards the granite system minimum these liquids can evolve into metaluminous hastingsite-biotite granite and peraluminous biotite granite compositions. These experiments show that partial melting of charnockitic monzonites cannot yield peralkaline liquids at the specified conditions (750–850° C).Syenitic rocks of this province may either reflect the action of late stage albitising fluids on residual charnockitic material after partial melt extraction or the collection of feldspar floated from a hastingsite-fayalite granite liquid.  相似文献   

2.
Petrogenetic grids are a powerful tool for understanding metamorphic terrains and many theoretical grids have been suggested for the process of granulite formation in metapelitic rocks, via fluid-absent biotite melting reactions. However, application of these grids has been difficult due to the lack of suitable experimental constraints. We present here an experimentally determined and tightly constrained petrogenetic grid for KFMASH system metapelites which extends from 840–1000°C and 5.0–12.5 kbar. Sixty four experiments on three KFMASH, mineral-mix, bulk compositions (X Mg=0.62, 0.74, 0.86) provide phase composition and assemblage data from which a grid can be derived and constrained. Reversal experiments and consideration of the phase composition data show the experiments to be close to equilibrium. The KFMASH univariant fluid-absent biotite melting reactions occur between 850 and 870°C at 5 kbar and between 900 and 915°C at 10 kbar. These reactions are connected to equilibria beyond the stability of biotite to develop a fixed framework within which the phase assemblage evolution of metapelitic rocks can be interpreted. The effect of minor components on phase equilibria is evaluated using the experimentally determined grid as a simple-system reference. The temperature at which melting occurs in metapelites is strongly controlled by the concentrations of titanium and fluorine in biotite. Pressure-temperature pseudosections presented for each of the experimental compositions show both the univariant and divariant reactions available to a particular bulk composition, clearly illustrating the possible evolution of the phase assemblage. The pseudosections also constrain the stability limits of  相似文献   

3.
E. Hegner  M. Klbl-Ebert  J. Loeschke 《Lithos》1998,45(1-4):395-411
Subsequent to high-pressure and temperature metamorphism of the axial zone of the Variscan foldbelt in central Europe at ca. 340 to 330 Ma, formation of lamprophyre dikes during transtensional tectonics may be viewed as the beginning of the post-collisional stage of the orogeny. We report the results of 40Ar/39Ar mica dating, major and trace element data, and isotope compositions for lamprophyre and rhyodacite samples from the southern Black Forest. The chemical compositions of these rocks shed light on the upper mantle and crust at the end of the Variscan orogeny. 40Ar/39Ar plateau ages for four phlogopite–biotite separates from lamprophyres indicate emplacement at 332 to 314 Ma. This event coincides with melting of the crust as indicated by 40Ar/39Ar biotite plateau ages of ca. 332 Ma for rhyodacite dikes which are probably related to coeval undeformed granites. Incompatible trace element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of sediment in a subduction-modified mantle. Nd, Sr, and Pb isotope compositions indicate an enriched mantle source with Nd-values of −1.5 to −6.8 which is similar to Variscan crust. Significant contamination of the lamprophyric melts by Variscan crust can be ruled out as mantle-derived phlogopite phenocrysts have similar Nd-values as in the whole-rock samples. We propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melted sediment. The isotope compositions of Variscan lamprophyres from western Europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust, as has been invoked for the origin of post-collisional granites. The isotope compositions of Tertiary basalts and mantle xenoliths indicate a depleted upper mantle under western Europe, implying that the enriched Variscan material was efficiently removed and mixed into the convecting mantle.  相似文献   

4.
K–Ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the Semail Ophiolite. The hornblende ages of 99±0.5 and 102±0.8 Ma and the 90 Ma ages of coexisting micas from the high-grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism (T 800–850° C, P 6.5–9 kbar). The new pressure estimates are based on findings of kyanite in garnet-amphibolite and cordierite in quartzitic rocks. These data indicate a cooling rate of 10–30° C/Ma. The oldest mica ages of 95±1 Ma are observed in the lowest-grade greenschists. These also largely represent cooling ages, but might in part also include formation ages. The pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low-grade metamorphic zone, through the high-grade rocks and into the base of the overlying ophiolite. Radiometric ages of hornblendes (92.3±0.5 and 94.8±0.6 Ma) indicate that the crustal gabbro sequence cooled below 500° C later than the base of the ophiolite sequence. Metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate. The peak of metamorphism must have been contemporaneous with the main magmatism in the Semail Ophiolite. One of the dated muscovites yields an age of 81.3±0.8 Ma, but this is related to discrete deformation zones that were active during late-stage emplacement of the ophiolite.  相似文献   

5.
By comparison with the general features of metamorphic soles (e.g. vertical and lateral extension, metamorphic grade and diagnostic mineral parageneses, deformation and dominant rock types), it is inferred that the amphibolites, metagabbros and hornblendites of the Wadi Um Ghalaga–Wadi Haimur area in the southern part of the Eastern Desert of Egypt represent the metamorphic sole of the Wadi Haimur ophiolite belt. The overlying ultramafic rocks represent overthrusted mantle peridotite. Mineral compositions and thermobarometric studies indicate that the rocks of the metamorphic sole record metamorphic conditions typical of such an environment. The highest P – T conditions ( c . 700 °C and 6.5–8.5 kbar) are preserved in clinopyroxene amphibolites and garnet amphibolites from the top of the metamorphic sole, which is exposed in the southern part of the study area. The massive amphibolites and metagabbros further north (Wadi Haimur) represent the basal parts of the sole and show the lowest P – T  conditions (450–620 °C and 4.7–7.8 kbar). The sole is the product of dynamothermal metamorphism associated with the tectonic displacement of ultramafic rocks. Heat was derived mainly from the hot overlying mantle peridotites, and an inverted P – T  gradient was caused by dynamic shearing during ophiolite emplacement. Sm/Nd dating of whole-rock–metamorphic mineral pairs yields similar ages of c . 630 Ma for clinopyroxene and hornblende, which is interpreted as a lower age limit for ophiolite formation and an upper age limit for metamorphism. A younger Sm/Nd age for a garnet-bearing rock ( c . 590 Ma) is interpreted as reflecting a meaningful cooling age close to the metamorphic peak. Hornblende K/Ar ages in the range 570–550 Ma may reflect thermal events during late orogenic granite magmatism.  相似文献   

6.
Fluid-absent melting experiments on a biotite (20 wt.%) andhornblende (2 wt.%) bearing tonalitic gneiss were conductedat 6 kbar (900–975C), 10 kbar (875–1075C), and14 kbar (950–975C) to study melt productivity from weaklyperaluminous quartzofeldspathic metamorphic rocks. At 6 kbar,biotite dehydration–melting is completed at 975C viaincongruent melting reactions that produce orthopyroxene, twooxides, and {small tilde}25 wt.% granitic melt. At 6 kbar, hornblendedisappears at 900C, probably in reaction with biotite. At 10kbar, biotite dehydration–melting produces <10 wt.%melt up to 950C via incongruent melting reactions that produceorthopyroxene, garnet, and granitic melt. Hornblende disappearsin the satne temperature interval either by resorption or byreaction with biotite. Widespread biotite dehydration–meltingoccurs between 950 and 975C and produces orthopyroxene, twooxides, and {small tilde}20 wt.% fluorine-rich (up to 0•31wt.%) granitic melt. At 14 kbar only a trace of melt is presentat 950C, and the amounts of hornblende and biotite are virtuallythe same as in the starting material. At 975C, hornblende isgone and {small tilde}10 wt.% granitic melt is produced by meltingof both biotite and hornblende. Our results show that hornblende-bearing assemblages cannotgo through dehydration–melting on their own (althoughthey can in combination with biotite) if the Ca content in thesource rock is too low to stabilize clinopyroxene. In such rocks,hornblende will either resorb or melt by reaction with biotite.Under fluid-absent conditions, intrusion of hot, mantle-derivedmagmas into the lower crust is necessary to initiate widespreaddehydration–melting in rocks with compositions similarto those discussed here. We argue that the high thermal stabilityof biotite in our starting material is caused mainly by theincorporation of fluorine. The relatively high F content inbiotite in the starting material (0•47 wt.%) suggests thatthe rock has experienced dehydroxylation in its past. F enrichmentby a previous fluid-absent partial melting event is excludedbecause of the lack of phases such as orthopyroxene and garnetwhich would have been produced. Our experiments show that thedehydration–melting of such F-enriched biotite producesF-rich granitic liquids, with compositions within the rangeof A-types granites, and leaves behind a granulitic residuedominated by orthopyroxene, quartz, and plagioclase. This studytherefore supports the notion that A-type granites can be generatedby H2O-undersaturated melting of rocks of tonalitic composition(Creaser et al., 1991), but does not require that these sourcerocks should be residual after a previous melting event.  相似文献   

7.
The Miocene Kitami rhyolite, consisting of orthopyroxene and plagioclase-phyric lavas and dikes, occurs on the back-arc side of the Kuril arc with coeval basalts and Fe-rich andesites. Temperatures estimated from orthopyroxene–ilmenite pairs exceed 900°C. Although the whole rock compositions of the Kitami rhyolite correspond to S-type granites (i.e., high K, Al, large ion lithophile elements, and low Ca and Sr), Sr–Nd isotope compositions are remarkably primitive, and similar to those of the coeval basalts and andesites. They are distinct from those of lower crustal metamorphic rocks exposed in the area. Comparison of chondrite-normalized rare earth element (REE) patterns between the rhyolite and the basalts and andesites show that the rhyolite is more light REE enriched, but has similar heavy REE contents than the basalts. All rhyolites show negative Eu anomalies. The geochemical data suggest that did not formed by simple dehydration melting of basaltic rocks or fractional crystallization of basaltic magmas. The features of slab-derived fluids expected from recent high pressure experimental studies indicates that mantle wedge is partly metasomatized with “rhyolitic” materials from subducted slabs; it is more likely that very low degree partial melting of the metasomatized mantle wedge formed the rhyolite magma.  相似文献   

8.
Decompression melting under a spreading centre should generate a residual upper mantle section with a weak vertical gradient in the degree of chemical depletion. Chemical data from the 6 km-thick ophiolitic mantle section of the Table Mountain massif (Bay of Islands Ophiolite, Newfoundland) reveal, however, a depletion gradient of 1% melting/100 m in the basal mantle section of the massif. Within the framework of a segmented ridge we suggest that during ophiolite detachment intermittent accretion of progressively less depleted peridotites occurred to the (former) base of the ophiolite, amounting to a several hundred metre-thick lherzolitic sole. In this view, the basal mantle section in Table Mountain extends a structural concept previously established for the metamorphic sole: both metamorphic gradient in the sole and melting gradient in the basal mantle rocks are apparent and of tectonic nature. Like the sole, the basal mantle sequence may thus expose mantle material derived from a scale larger than the preserved ophiolite complex.  相似文献   

9.
Melting experiments on a mantle-derived nodule assemblage consisting of clinopyroxene, phlogopite and minor titanomagnetite, sphene and apatite have been done at 20 and 30 kbar between 1,175 and 1,300° C. The nodule composition was selected on the basis of modal and chemical analyses of 84 mantle derived nodules with metasomatic textures from the Katwe-Kikorongo and Bunyaruguru volcanic fields of south-west Uganda. At 30 kbar, 1,225 and 1,250° C, representing 20–30% partial melting, the compositions of glasses compare favourably to those of the average composition of 26 high potassic mafic lavas from the same region. Glasses produced by sufficiently low degrees of partial melting at 20 kbar could not be analysed. Glass compositions obtained for 20–30% melting at 30 kbar have high K2O (3.07–5.05 wt.%), low SiO2 (35.0–39.2 wt.%), high K/K + Na (0.54–0.71), K + Na/Al (0.99–1.08) and Mg/ Mg + FeT of 0.59–0.62. These results support the suggestion of Lloyd and Bailey (1975) that the nodules represent the source material for the high K-rich lavas of south-west Uganda. If this conclusion is correct it implies that anomalous mantle source of phlogopite clinopyroxenite composition could produced the Ugandan lavas by relatively higher degrees of partial melting than that normally considered for highly alkaline mafic magmas derived from a pyrolitic mantle source. Higher degrees of melting are considered likely from such a different source region, rich in alkalis, water and radioactive elements. Steeper geotherms and increased fluxing of sub-rift mantle by degassing would also produce higher degrees of partial melting.  相似文献   

10.
陕西陈家庄铀矿床是我国北秦岭商州—丹凤伟晶岩型铀矿集区中一个重要的矿床,铀矿体均产于加里东期花岗岩体周边花岗伟晶岩脉与围岩(秦岭群变质杂岩)的接触部位。本文对矿区花岗岩体、花岗伟晶岩脉开展了详细的岩石学、岩石地球化学、锆石U-Pb年代学研究,进而对其成因、成岩构造环境和铀矿化机理进行了探讨。LA-ICP-MS锆石U-Pb年代学研究表明,黄龙庙黑云母花岗岩体,陈家庄二长花岗岩体和非矿、贫矿、富矿花岗伟晶岩脉的成岩年龄分别为(446±3) Ma、(419±2) Ma、(417±3) Ma、(414±4) Ma和(416±3) Ma。地球化学分析显示：黄龙庙黑云母花岗岩体具有Ⅰ型花岗岩、埃达克质岩特征,源自加厚下地壳的部分熔融,形成于块体碰撞构造环境;陈家庄二长花岗岩体也具有I型花岗岩特征,但源区深度略浅,形成于碰撞后的减压环境。花岗伟晶岩脉与陈家庄二长花岗岩体近于同时形成,且具有亲缘性。铀矿物及富铀黑云母均产于花岗伟晶岩脉中。对比研究揭示,非矿、贫矿、富矿花岗伟晶岩脉地球化学特征和铀赋存状况的差异由同化混染作用程度高低所致。在花岗伟晶岩脉与秦岭群变质杂岩的接触部位,同化混染作用较弱的部位形成的二云母花岗伟晶岩脉仅具有弱的铀富集,同化混染作用较强的部位所形成的富石英、黑云母花岗伟晶岩脉则高度富集铀且构成铀矿体。综合研究表明,花岗伟晶岩脉成岩期后的同化混染作用是铀富集成矿的主导因素。  相似文献   

11.
马遥  刘学飞  梁亚运  杨溢 《岩石学报》2019,35(5):1566-1582
胶东地区广泛发育早白垩世中-酸性脉岩群,但是其成因演化及成岩地质背景至今仍存在诸多争论。本文利用电子探针(EMPA)与激光剥蚀电感藕合等离子质谱(LA-ICP-MS)技术分析了胶东早白垩世石英闪长脉岩与闪长脉岩中主要造岩矿物(斜长石和黑云母)的主、微量元素组成;并结合岩石地球化学特征,对两者的岩浆源区和岩浆演化进行了研究探讨。石英闪长脉岩与闪长脉岩中黑云母低于检测线的Ca O含量与斜长石主量元素之间良好的线性关系指示两者为未受到后期变质作用影响的原生矿物,进一步说明胶东中生代石英闪长脉岩与闪长脉岩岩浆形成后,在上涌成岩过程中未受到变质作用的影响。石英闪长脉岩中壳源黑云母矿物成分基本一致的,以及斜长石正环带中核边部线性变化的An值与Fe、Mg、Sr、Ba等不相容元素特征指示石英闪长脉岩源于华北克拉通东部古老的加厚下地壳部分熔融作用,并在岩浆演化早期和晚期有一定幔源镁铁质岩浆混入,整个岩浆演化过程并未受到大气、俯冲、变质流体混入或构造作用的影响。闪长脉岩中黑云母矿物较大的Fe2+/(Fe2++Mg)比值范围,Al含量与结晶压力高度正相关以及斜长石中不相容元素特征指示本次研究中胶东闪长脉岩源自俯冲的板片来源的流体或沉积物混入所形成富集岩石圈地幔源区。胶东早白垩世石英闪长脉岩与闪长脉岩形成的大地构造动力学背景为古太平洋板俯冲-回撤引起热-机械侵蚀,进而导致岩石圈地幔减薄。在此情况下软流圈地幔上涌加热导致胶东富集岩石圈地幔部分熔融形成地幔熔体。这些幔源熔体经历分离结晶形成早白垩世闪长脉岩。此外,幔源镁铁质岩浆持续加热导致加厚下地壳部分熔融,形成了石英闪长脉岩。  相似文献   

12.
A number of Cu-Zn-Fe-Pb-As sulfide mineralizations spatially associated with metabasic sills, dikes, and flows of originally continental tholeiitic composition occur in the 1.9–1.8-Ga-old metavolcano-sedimentary sequence in the Sax» sedimentary rift basin. In higher stratigraphic levels weak sulfide mineralizations occur just below Narich, albite-chlorite-calcite-quartz-actinolite-bearing metabasic flows; in the lower stratigraphic levels sulfide mineralizations occur (a) spatially associated with potassiumrich, microcline-scapolite- or biotite/phlogopite-rich metabasic sills and dikes, and (b) below metabasic flows in which albite and calcite are replaced by microcline and scapolite. The sodium- and potassium-rich metabasites are demonstrated to represent products of spilitization and potassic alteration, respectively. During spilitization Fe, Cu, Co, and HREE are leached from the basic flows, whereas during potassic alteration Fe, Cu, Y, and possibly Zn and Zr are leached from the basic rocks, indicating a genetic relationship between the metabasites and the Cu-Fe-sulfide mineralizations. Spilitization probably took place due to interaction of the flows with descending, sea-water-dominated fluids in a subseafloor hydrothermal system with temperatures roughly between 250° and 400°C. Potassic alteration probably took place by ascending fluids at higher temperatures than for spilitization.  相似文献   

13.
Ophiolites are widespread across the Anatolian plate and formed during the termination of different branches of the Neotethys ocean. The Tekirova ophiolite, situated in the Antalya ophiolite nappes in Southwest Anatolia, originated in the southern branch of the Neotethys. The former ophiolite consists of mantle and crustal sequences, in which, it is predominantly crosscut by the isolated diabasic dikes. Here, we have distinguished two dike types on the basis of their petrographic features and also degree of alteration. The relatively fresh types are generally named as diabase dikes. While the altered types are called as metasomatised dikes, and these dikes display ophitic to subophitic textures. The diabase and metasomatised dikes show enrichment in LILEs and depletion in HFSEs, indicating characteristics of suprasubduction zone ophiolites. This depletion of mobile elements may have been caused by metasomatism. The HFSEs versus HREEs diagrams show that the dikes were formed from 10 to 30% melting of extracted melts from previous mantle melting in which modified by subduction-derived fluids.  相似文献   

14.
To date, few adakitic rocks have been reported in direct association with contemporary intra-continental extensional structures, which has cast doubt on genetic models involving partial melting of the lower crust. This study presents Early Cretaceous (143-129 Ma, new Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages) adakitic granites, which are directly associated with a contemporary metamorphic core complex (i.e., the Northern Dabie Complex in the Dabie area). These granites exhibit relatively high Sr contents, negligible to positive Eu and Sr anomalies, high La/Yb and Sr/Y ratios, but very low Yb and Y contents, similar to subducted oceanic crust-derived adakites. They are also characterized, however, by very low MgO or Mg# and Ni values, and Nd-Sr isotope compositions (εNd(t) = −14.6 to −19.4 and (87Sr/86Sr)i = 0.7067-0.7087) similar to Triassic continent-derived eclogites subducted in the Dabie-Sulu Orogen. Additionally, late granitic dikes in the adakitic intrusions exhibit low Sr contents, clearly negative Eu and Sr anomalies, low La/Yb and Sr/Y ratios, but relatively high Yb and Y contents, similar to 118-105 Ma granites in the Northern Dabie Complex. Based on composition and geochronology data of Neoproterozoic amphibolites and orthogneisses, Triassic high- to ultra-high pressure metamorphic rocks, and Early Cretaceous mafic-ultramafic intrusive rocks, and the constraints provided by experimental melt data for tonalites, metabasaltic rocks and eclogites, we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile-bearing eclogitic lower crust as a consequence of Triassic-Middle Jurassic subduction and thrusting. The late dikes probably originated from plagioclase-bearing intermediate granulites. Moreover, we suggest that late Mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of Early Cretaceous mafic-ultramafic intrusive rocks in the Dabie area, and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite. Asthenospheric upwelling in response to post-collisional delamination of lithospheric mantle was likely to have provided the heat source for the Cretaceous magmatism.  相似文献   

15.
Lycian ophiolites located in the Western Taurides, are cut at all structural levels by dolerite and gabbro dikes. The dolerite dikes from this area are both pristine and metamorphosed. The non-metamorphosed dikes are observed both in the peridotites and in the metamorphic sole rocks. Accordingly, the non-metamorphosed dikes cutting the metamorphic sole were generated after cooling of the metamorphic sole rocks. The metamorphosed dolerite dikes are only observed in the peridotites. The physical conditions and timing of the metamorphism for the metamorphosed dolerite dikes are similar to those of the metamorphic sole rocks of the Lycian ophiolites suggesting that the metamorphosed dolerite dikes were metamorphosed together with the metamorphic sole rocks. Therefore, the dike injections in the western part of the Tauride Belt Ophiolites occurred before and after the generation of the metamorphic sole rocks. All metamorphosed and non-metamorphosed dikes are considered to have the same origin and all of them are subduction-related as inferred from whole-rock geochemistry and lead isotopes. Lead isotope compositions of whole rocks of both dike groups cluster in a narrow field in conventional Pb isotope diagrams (206Pb/204Pb = 18.40–18.64; 207Pb/204Pb = 15.56–15.58; 208Pb/204Pb = 38.23–38.56) indicating a derivation from an isotopically homogeneous source. On the 207Pb/204Pb versus 206Pb/204Pb diagram, isotope compositions of the dikes plot slightly below the orogen curve suggesting contributions from mantle reservoir enriched by subducted oceanic lithosphere. Such a signature is typical of island arc magmatic rocks and supports the formation of the investigated rocks in a subduction-related environment.  相似文献   

16.
The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.  相似文献   

17.
The Semail ophiolite located in the eastern part of the Arabian platform preserves remnants of ocean plate stratigraphy and related metamorphic sole. To understand the petro-tectonic evolution of a metamorphic sole during subduction to obduction processes, here we investigate the garnet metagabbros from the metamorphic sole and the tonalites which intruded the mantle section of the Khor Fakkan Block. We present results from petrology, geochemistry, zircon U-Pb, Hf and O isotope analyses and phase equilibria modeling. The garnet metagabbro samples have E-MORB-type enriched-mantle compositions with zircon dates of ca. 89–96 Ma, and positive εHf(t) values ranging from 5.6 to 10.0. The tonalite is peraluminous with those range of ca. 87–92 Ma, and a range of positive εHf(t) values of 5.1–10.0. The similarity in εHf values from both the garnet metagabbro and tonalite samples suggests a strong relevance to their mantle source, indicating the role of subducted material during their formation. In contrast, the δ18O(zircon) values show distinctly different values of high δ18O(zircon) of ~13–16‰ for the tonalite and ~ 5–8‰ for the metagabbro samples, reflecting variations in the role of surface-derived source materials. The phase equilibria modeling of the garnet metagabbro shows high-pressure amphibolite facies metamorphism that preceded the peak granulite facies metamorphism, followed by lower pressure hydration and decompression. This clockwise P-T path might reflect partial melting and differentiation of mantle wedge section above subducted slab. Our results provide insights into the complex processes within a supra-subduction zone, implying differences in degree of partial melting of the ocean plate stratigraphic sequences including recycled oceanic slab and surface-derived marine sediments that were subsequently interacted with hydrothermally altered mantle at a mantle wedge during subduction to obduction processes that formed the Semail ophiolite during the Upper Cretaceous.  相似文献   

18.
The Mallee Bore area in the northern Harts Range of central Australia underwent high-temperature, medium- to high-pressure granulite facies metamorphism. Individual geothermometers and geobarometers and average P–T  calculations using the program Thermocalc suggest that peak metamorphic conditions were 705–810 °C and 8–12 kbar. Partial melting of both metasedimentary and meta-igneous rocks, forming garnet-bearing restites, occurred under peak metamorphic conditions. Comparison with partial melting experiments suggests that vapour-absent melting in metabasic and metapelitic rocks with compositions close to those of rocks in the Mallee Bore area occurs at 800–875 °C and >9–10 kbar. The lower temperatures obtained from geothermometry imply that mineral compositions were reset during cooling. Following the metamorphic peak, the rocks underwent local mylonitization at 680–730 °C and 5.8–7.7 kbar. After mylonitization ceased, garnet retrogressed locally to biotite, which was probably caused by fluids exsolving from crystallizing melts. These three events are interpreted as different stages of a single, continuous, clockwise P–T  path. The metamorphism at Mallee Bore probably occurred during the 1745–1730 Ma Late Strangways Orogeny, and the area escaped significant crustal reworking during the Anmatjira and Alice Springs events that locally reached amphibolite facies conditions elsewhere in the Harts Ranges.  相似文献   

19.
Fluid-solid-solid dihedral angles in the NaCl-H2O-CO2-calcite-dolomite-magnesite system have been determined at pressures ranging from 0.5 to 7 kbar and temperatures from 450°C to 750°C. At 1 kbar and 650°C, both dolomite and magnesite exhibit a dihedral angle minimum for intermediate H2O-CO2 fluids similar to that previously determined by the present authors for calcite, but the depth of the minimum is smaller, being above the critical value of 60° for both dolomite and magnesite for all fluid compositions. Calcite-calcite-brine dihedral angles at 650°C have been determined in the pressure range 1–5 kbar. Angles decrease with increasing salt content of the fluid, tending towards a constant value of about 65° for strong brines at pressures above 2 kbar. There is a general increase of angle with increasing pressure which is most marked for strong brines. A positive correlation of angle with pressure is also observed in calcite-H2O-CO2 fluids, the position of the minimum moving towards higher angles and towards H2O-rich fluids with increasing pressure. The permeability window previously observed by the present authors at 1 kbar and intermediate fluid compositions closes at about 1.5 kbar. The results demonstrate that the permeability of carbonates to grain edge fluid flow is only possible at low pressures and for fluids of restricted H2O-CO2-NaCl compositions. However, geochemical evidence from metamorphic terrains suggests that pervasive infiltration does occur under conditions where impermeability is predicted. From examination of published studies of infiltrated carbonates we conclude that deformation plays a critical role in enhancing carbonate permeability. Possible mechanisms for this include shear-enhanced dilatancy (micro-cracking), fluid inclusion drag by deformation-controlled grain boundary migration, and dynamically maintained transient grain boundary fluid films.  相似文献   

20.
The mineralogy, petrology and geochemistry of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare-element pegmatites. The granite was emplaced at 3–4 kbar as multiple sills and dikes into quartz-mica schists at the culmination of a regional high-temperature, low-pressure metamorphic event. Principally along the periphery of the main pluton and in satellite intrusions, the sills segregated into granite-pegmatite couplets. The major minerals include quartz, K-feldspar, sodic plagioclase and muscovite. Biotite-{Mg No. [Molar MgO/(MgO+FeO)]=0.32-0.38} is the predominant ferromagnesian mineral in the granite's core, whereas at the periphery of the main pluton and in the satellite intrusions tourmaline (Mg No.=0.18–0.48) is the dominant ferromagnesian phase. Almandine-spessartine garnet is also found in the outer intrusions. There is virtually a complete overlap in the wide concentration ranges of SiO2, CaO, MgO, FeO, Sr, Zr, W of the biotite- and tourmaline-bearing granite suites with no discernable differentiation trends on Harker diagrams, precluding the derivation of one suite from the other by differentiation following emplacement. This is consistent with the oxygen isotope compositions which are 11.5 ± 0.6 for the biotite granites and 13.2 ± 0.8 for the tourmaline granites, suggesting derivation from different sources. The concentrations of TiO2 and possibly Ba are higher and of MnO and B are lower in the biotite granites. The normative Orthoclase/Albite ratio is extremely variable ranging from 0.26 to 1.65 in the biotite granites to 0.01–1.75 in the tourmaline granites. Very few sample compositions fall near the high-pressure, watersaturated haplogranite minima-eutectic trend, indicating that the granites for the most part are not minimum melts generated under conditions with =1. Instead, most biotite granites are more potassic than the water-saturated minima and eutectics and in analogy with experimentally produced granitic melts, they are best explained by melting at 6 kbar, <1 and temperatures 800°C. Such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals (Nabelek et al. 1992). Several of the tourmaline granite samples contain virtually no K-feldspar and have oxygen isotope equilibration temperatures 716–775°C. Therefore, they must represent high-temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water-saturated haplogranite minima or during fractionation of intruded melts into granite-pegmatite couplets accompanied by volatile-aided differentiation of the alkali elements. The indicated high temperatures, <1, the relatively high TiO2 and Ba concentrations and the relatively low values of the biotite granites suggest that they were generated by high-extent, biotite-dehydration melting of an immature Archean metasedimentary source. The ascent of the hot melts may have triggered low-extent, muscovite-dehydration melting of schists higher in the crust producing the high-B, low-Ti melts comprising the periphery of the main pluton and the satellite intrusions. Alternatively, the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse, with the low- biotite granites generated at a deeper, hotter region and the high- tourmaline granites at a higher, cooler region of the crust. The low-Ti and high-B concentrations in the high- melts resulted in the crystallization of tourmaline rather than biotite, which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions.  相似文献   

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