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1.
The G?ksun(Kahramanmaras)ophiolite(GKO),cropping out in a tectonic window bounded by the Malatya metamorphic unit on both the north and south,is located in the EW-trending lower nappe zone of the southeast Anatolian orogenic belt(Turkey).It exhibits a complete oceanic lithospheric section and overlies the Middle Eocene Maden Group/Complex with a tectonic contact at its base.The ophiolitic rocks and the tectonically overlying Malatya metamorphic(continental)unit were intruded by I-type calc-alkaline Late Cretaceous granitoid(~81-84 Ma).The ultramafic to cumulates in the GKO are represented by wehrlite,plagioclase wehrlite,olivine gabbro and gabbro.The crystallization order for the cumulate rocks is as follows:olivine±chromian spinel→clinopyroxene→plagioclase.The major and trace element geochemistry as well as the mineral chemistry of the ultramafic to mafic cumulate rocks suggest that the primary magma generating the GKO is compositionally similar to that observed in the modern island-arc tholeiitic sequences.The mineral chemistry of the ultramafic to mafic cumulates indicates that they were derived from a mantle source that was previously depleted by earlier partial melting events.The highly magnesian olivine(Fo77-83),clinopyroxene(Mg#of 82-90)and the highly Ca-plagioclase(An81-89)exhibit a close similarity to those,which formed in a supra-subduction zone(SSZ)setting.The field and the geochemical evidence suggest that the GKO formed as part of a much larger sheet of oceanic lithosphere,which accreted to the base of the Tauride active continental margin,including the ispendere,K?mürhan and the Guleman ophiolites.The latter were contemporaneous and genetically/tectonically related within the same SSZ setting during the closure of the Neotethyan oceanic basin(Berit Ocean)between the Taurides to the north and the Bitlis-Pütürge massif to the south during the Late Cretaceous.  相似文献   

2.
A mafic–ultramafic intrusive belt comprising Silurian arc gabbroic rocks and Early Permian mafic–ultramafic intrusions was recently identified in the western part of the East Tianshan, NW China. This paper discusses the petrogenesis of the mafic–ultramafic rocks in this belt and intends to understand Phanerozoic crust growth through basaltic magmatism occurring in an island arc and intraplate extensional tectonic setting in the Chinese Tianshan Orogenic Belt (CTOB). The Silurian gabbroic rocks comprise troctolite, olivine gabbro, and leucogabbro enclosed by Early Permian diorites. SHRIMP II U-Pb zircon dating yields a 427 ± 7.3 Ma age for the Silurian gabbroic rocks and a 280.9 ± 3.1 Ma age for the surrounding diorite. These gabbroic rocks are direct products of mantle basaltic magmas generated by flux melting of the hydrous mantle wedge over subduction zone during Silurian subduction in the CTOB. The arc signature of the basaltic magmas receives support from incompatible trace elements in olivine gabbro and leucogabbro, which display enrichment in large ion lithophile elements and prominent depletion in Nb and Ta with higher U/Th and lower Ce/Pb and Nb/Ta ratios than MORBs and OIBs. The hydrous nature of the arc magmas are corroborated by the Silurian gabbroic rocks with a cumulate texture comprising hornblende cumulates and extremely calcic plagioclase (An up to 99 mol%). Troctolite is a hybrid rock, and its formation is related to the reaction of the hydrous basaltic magmas with a former arc olivine-diallage matrix which suggests multiple arc basaltic magmatism in the Early Paleozoic. The Early Permian mafic–ultramafic intrusions in this belt comprise ultramafic rocks and evolved hornblende gabbro resulting from differentiation of a basaltic magma underplated in an intraplate extensional tectonic setting, and this model would apply to coeval mafic–ultramafic intrusions in the CTOB. Presence of Silurian gabbroic rocks as well as pervasively distributed arc felsic plutons in the CTOB suggest active crust-mantle magmatism in the Silurian, which has contributed to crustal growth by (1) serving as heat sources that remelted former arc crust to generate arc plutons, (2) addition of a mantle component to the arc plutons by magma mixing, and (3) transport of mantle materials to form new lower or middle crust. Mafic–ultramafic intrusions and their spatiotemporal A-type granites during Early Permian to Triassic intraplate extension are intrusive counterparts of the contemporaneous bimodal volcanic rocks in the CTOB. Basaltic underplating in this temporal interval contributed to crustal growth in a vertical form, including adding mantle materials to lower or middle crust by intracrustal differentiation and remelting Early-Paleozoic formed arc crust in the CTOB.  相似文献   

3.
The Jurassic Bonanza arc, on Vancouver Island, British Columbia, represents an exhumed island arc crustal section of broadly diorite composition. We studied bodies of mafic and ultramafic cumulates within deeper levels of the arc to constrain the conditions and fractionation pathways leading from high-Mg basalt to andesite and dacite. Major element trends coupled with textural information show the intercumulus crystallization of amphibole, as large oikocrysts enclosing olivine in primitive cumulates controls the compositions of liquids until the onset of plagioclase crystallization. This process is cryptic, occurring only in the plutonic section, and explains the paucity of amphibole in mafic arc volcanics and the change in the Dy/Yb ratios in many arc suites with differentiation. The correlation of octahedral Al in hornblende with pressure in liquidus experiments on high-Mg basalts is applied as an empirical barometer to hornblendes from the Bonanza arc. It shows that crystallization took place at 470–880 MPa in H2O-saturated primitive basaltic magmas. There are no magmatic equivalents to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will shift the bulk arc composition to the high-Mg# andesite composition of bulk continental crust. Garnet removal from wet magmas appears to be the key factor in producing continental crust, requiring high pressures and thick crust. Because oceanic island arcs are built on thinner crust, the long-term process generating the bulk continental crust is the accretion of island arcs to continental margins with attendant tectonic thickening.  相似文献   

4.
Mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35% olivine, occur at three different levels in Olivine-Bearing zones III and IV of the Middle Banded series of the Stillwater complex. The discordant bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses. Although the host olivine-gabbros are foliated with tabular plagioclase, the discordant bodies lack a discernible fabric and have blocky plagioclase. Average olivine in the host rocks is slightly more magnesian than that of the discordant bodies (Mg#75.8 ± 0.7 versus Mg#74.6 ± 0.3 respectively) but plagioclase compositions are indistinguishable (An77.6 ± 2.0 versus An76.6 ± 4.3– average host and discordant bodies respectively). Whole-rock major- and trace-element compositions of the discordant bodies are generally indistinguishable from cumulates with similar modal abundance. However, bulk compositions of anorthositic cores from the discordant bodies are enriched in K, Na, Ba, Sr and P. We conclude that the discordant bodies formed when cooler volatile fluids or fluid-rich silicate liquids moved upward and encountered a hotter undersaturated solid-plus-liquid assemblage. Continued liquid/fluid fluxing increased the permeability along the flow path and focused the flow, allowing the original bulk compositions to be modified and leaving plagioclase-rich troctolites and anorthosites. The shapes of the discordant bodies suggest that the cumulus pile had anisotropic permeability during late-stage liquid/fluid flow. Chemical and mineralogical evidence from other parts of Olivine-Bearing zones III and IV suggests that the processes that formed the discordant bodies may have influenced other cumulates. In fact, it appears that the same processes that formed the discordant bodies operated within an anorthositic layer, strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence. Received: 10 December 1996 / Accepted: 12 August 1997  相似文献   

5.
Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.  相似文献   

6.
The Archean Shawmere Anorthosite Complex, at the southern end of the Kapuskasing Structural Zone, consists dominantly of anorthosite (An65 –85) with minor gabbroic and ultramafic units, which are completely enclosed and cut by tonalites. Both the anorthosites and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the rocks have undergone high grade metamorphism and are recrystallized so that few igneous textures remain.The anorthosites, gabbros and ultramafic rocks of this complex are cumulates which contain calcic plagioclase (An65–95) and have atomic Mg/(Mg + Fe2+) ratios (Mg#) greater than 0.6; less than 3 ppm Rb; 150–210 ppm Sr; and less than 60 ppm Ba. REE abundanees range from 0.2 to 10 times chondritic and exhibit both light-enriched and light-depleted REE patterns. The lower Mg# for the samples having more enriched light REE indicates substantial fractions of ferromagnesian minerals crystallized in addition to plagioclase during fractional crystallization, suggesting that the parent magma was basaltic, and not anorthositic. The ranges in Sr, Ba and REE abundances required for the magmas are typical of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere Anorthosite Complex may represent cumulates of a crustal-level magma chamber which could have been the immediate source of basic Archean volcanics.One gabbroic anorthositic dike sample has a steeply fractionalted REE pattern with heavy REE abundances less than chondrites and a large positive Eu anomaly. The proposed interpretations is that this rock formed by partial melting of mafic cumulates, perhaps those of the Shawmere Anorthosite Complex itself.  相似文献   

7.
The Mersin ophiolite is an ～6-km-thick oceanic crustal sequence in the central Taurides (southern Turkey), forming large outcrops on the southern flank of Bolkardag. It comprises a sequence of units and includes, in ascending structural order, the Mersin melange, subophiolitic metamorphics, tectonics, ultramafic and mafic cumulates, and pillow lavas interlayered with deep-marine sediments.

Thin beds of ophiolite-related metamorphics occupy a constant stratigraphic position at the base of the thick ultramafic unit and include a characteristic rock association of amphibolite and amphibolitic schist, epidote-amphibole schist, quartz-mica schist, calcschist, and marble, which exhibits a typical inverted metamorphic zonation from upper amphibolite to greenschist facies. The protoliths of these metamorphics are oceanic-island basaltic rocks and associated pelagic sediments. They are cut by a swarm of approximately E-W-trending, post-metamorphic, MORB-type tholeiite, microgabbroic-diabasic dikes, which are seen not to affect the structurally overlying harzburgites. The initial detachment of these metamorphics from the oceanic crust during closure of the Neotethyan ocean commenced in the late Cenomanian (96 to 92 Ma, 40Ar/39Ar hornblende ages).

The metamorphics have a well-developed migmatitic structure and metamorphic banding defined by alternating epidote-rich and carbonate layers. A pronounced regional foliation (S1), NW-SE-trending mineral lineation (L1), and first-order intrafolial folds (F1) were produced during their deformation. Imbricated structure also is typical in these metamorphics. The meso-structures-such as asymmetry of calcite-augen, vergence and asymmetry of F1 folds, general trend of brittle-ductile structures such as oblique-slip normal and thrust/reverse faults, and vergence of thrusts in the imbricate structure—all indicate unambiguous non-coaxial deformation and a shear sense of upper levels moving up to the NW. The structural evidence suggests that the Mersin ophiolite was obducted over the Bolkardag Mesozoic carbonates from the SE to the NW. The shear sense obtained from the subophiolitic metamorphics is compatible with the general trend and vergence of the large-scale thrust sheets that comprise the Mersin ophiolite.  相似文献   

8.
西藏吉定蛇绿岩地球化学特征及其构造指示意义   总被引：3，自引：2，他引：1  
陈根文  刘睿  夏斌  邓腾 《岩石学报》2015,31(9):2495-2507
吉定蛇绿岩位于雅鲁藏布江蛇绿岩带的中段,是该带保存较好的蛇绿岩之一,通过对该岩体的研究及与附近蛇绿岩剖面的对比有助于恢复早白垩世雅鲁藏布江蛇绿岩带的演化过程。吉定蛇绿岩包括玄武岩、辉绿岩、堆晶岩及地幔橄榄岩四个岩石单元。壳层岩石岩浆结晶顺序为:橄榄石→单斜辉石→斜长石,代表湿岩浆系统分异。吉定蛇绿岩壳层熔岩(玄武岩和辉绿岩)Ti O2含量为0.87%~1.45%,平均1.1%,与印度洋N-MORB玻璃(1.19%)相似。REE配分模式具有明显的LREE亏损特征,稀土配分模式与典型的大洋中脊玄武岩相似。但其微量元素蛛网图上表现为富集LILE,而亏损HFSE,并具有较高LILE/HFSE比值特征,与俯冲带上的(SSZ)蛇绿岩相似。蛇绿岩熔岩在岩石地球化学上表现出既亲MORB,又具部分IAB的特征。结合区域上大竹卡、得几等蛇绿岩岩石及地球化学资料对比分析,提出吉定蛇绿岩形成于在洋内俯冲带上发育起来的弧后盆地,并提出日喀则地区早白垩世洋壳演化的解释模式:雅鲁藏布江中段蛇绿岩至少包含三种组分特征的蛇绿岩体,其代表性剖面分别是吉定,得村和大竹卡,分别形成于近俯冲带的弧后盆地、弧前盆地和弧后盆地,这些洋壳共同组成早白垩世时期的与特提斯洋俯冲带斜交的一条分段发育的洋中脊。  相似文献   

9.
This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among several examples of mafic and ultramafic layered sequences in the Sierras Valle Fértil and La Huerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc in central-western Argentina. The layered sequence grew at deep crustal levels (20–25 km) within a mafic lower crust. The base of the layered body was detached during the tectonic uplift of the Famatinian lower crust, whereas the roof of the layered body is exposed in the eastern zone. In the inferred roof, olivine-bearing rocks vanish, cumulate textures are less frequent, and the igneous sequence becomes dominated by massive or thinly banded gabbronorites. Mainly based on the petrographic relationships, the inferred order of crystallization in the gabbroic and peridotitic layered sequence is: (1) Cr–Al-spinel + olivine, (2) Cr–Al-spinel + olivine + clinopyroxene + magnetite, (3) Cr–Al-spinel + olivine + plagioclase + magnetite ± orthopyroxene, and (4) Al-spinel + orthopyroxene + amphibole. A strong linear negative correlation between olivine and plagioclase modal proportions combined with field, petrographic and geochemical observations are used to demonstrate that the physical separation of olivine and plagioclase results in rock diversity at scales of a few centimeters to tens of meters. However, the composition of olivine (Fo ～ 0.81) and plagioclase (An > 94%) remains similar throughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, and display chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic and peridotitic layered rocks have trace element concentrations reflecting cumulates of early crystallizing minerals. The trace element patterns still retain the typical features of subduction-related arc magmatism, showing that the process of cumulate formation did not obscure the trace element signature of the parental magma. Using the composition of cumulus minerals and whole-rock chemical trends, we show that the parental magma was mafic (SiO2 ～ 48 wt.%) with Mg-number around 0.6, and hydrous. The oxygen fugacity (fO2) of the parental magma estimated between +0.8 and ?0.6 log fO2 units around the fayalite–magnetite–quartz (FMQ) buffer is also characteristic of primitive hydrous arc magmas. The initially high water content of the parental magma allowed amphibole to crystallize as an interstitial phase all over the crystallization evolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblage gives rise to the retention of many trace elements which would otherwise be incompatible with the mineral assemblage of mafic–ultramafic cumulates. This study shows that there exist strongly mafic and primitive magmas that are both generated and emplaced within the lower crustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentina cannot be regarded as a typical Andean-type tectono-magmatic setting.  相似文献   

10.
石板墩堆晶岩位于中祁连地块西段党河断裂带北侧,主要由橄榄岩、蛇纹石化橄辉岩和辉长岩组成,具有多旋回、多韵律层的产出特征。辉长岩LA ICP MS锆石U Pb年龄为(4865 ± 33) Ma。岩石地球化学结果显示,蛇纹石化橄辉岩和辉长岩配分型式十分相似,具有富集大离子亲石元素、亏损高场强元素、LREE相对富集、HREE平坦型分布以及正Eu异常(Eu/Eu*=097~304)的特点。研究结果表明,蛇纹石化橄辉岩、辉长岩为同源岩浆作用的产物,源区为被俯冲流体交代过的软流圈地幔,形成于火山弧环境,是在岩浆作用过程中不断发生堆晶作用,并在堆晶之后再次泵入混合大量新的玄武岩浆反复进行所形成。结合区域大地构造背景,认为中祁连西段是早古生代早期在残留的微陆块基础上形成的一个火山弧增生杂岩地体。  相似文献   

11.
Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs.  相似文献   

12.
The Massif du Sud is a large ophiolitic complex that crops out in the southern region of New Caledonia (SW Pacific). It is dominated by harzburgite tectonite that locally shows a transitional gradation to massive dunite up section. Clinopyroxene, orthopyroxene and plagioclase progressively appear in dunite up to the transition to layered wehrlite and orthopyroxene–gabbro. The dunite–wehrlite and wehrlite–gabbro contacts are parallel and the latter defines the paleo-Moho.Highly depleted modal, mineral and bulk rock compositions indicate that harzburgites are residues after high degrees (20–30%) of partial melting mainly in the spinel-stability field. Their relative enrichment in HFSE, LREE and MREE is due to re-equilibration of melting residues with percolating melts. Dunite formed in the Moho transition zone by reaction between residual mantle harzburgite and olivine-saturated melts that led to pyroxene dissolution and olivine precipitation. Rare clinopyroxene and plagioclase crystallized in interstitial melt pores of dunite from primitive, low-TiO2, ultra-depleted liquids with a geochemical signature transitional between those of island arc tholeiites and boninites.Ascending batches of relatively high-SiO2, ultra-depleted melts migrated through the Moho transition zone and generated wehrlite by olivine dissolution and crystallization of clinopyroxene, orthopyroxene and plagioclase in variable amounts. These liquids were more evolved and were produced by higher degrees of melting or from a more depleted source compared with melts that locally crystallized clinopyroxene in dunite. Ultra-depleted magmas, non-cogenetic with those that formed the Moho transition zone, ascended to the lower crust and generated gabbroic cumulates with subduction-related affinity. Thus, the ultramafic and mafic rocks in the Moho transition zone and lower crust of the Massif du Sud ophiolite are not products of fractional crystallization from a single magma-type but are the result of migration and accumulation of different melts in a multi-stage evolution.The record of high partial melting in the mantle section, and migration and accumulation of ultra-depleted subduction-related melts in the Moho transition zone and lower crust support that the Massif du Sud ophiolite is a portion of forearc lithosphere generated in an extensional regime during the early phases of the subduction zone evolution. Our results show the existence of different types of ultra-depleted melt compositions arriving at the Moho transition zone and lower crust of an infant intraoceanic paleo-arc. Ultra-depleted melts may thus be a significant component of the melt budget generated in oceanic spreading forearcs prior to aggregation and mixing of a large range of melt compositions in the crust.  相似文献   

13.
The Wingellina Hills intrusion is a small composite gabbroic/ultramaficintrusion and forms a tectonically dismembered segment of theUpper Proterozoic Giles complex in central Australia. Its 1600m of exposed magmatic stratigraphy formed in a continuouslyfractionating, periodically replenished magma chamber. Olivinegabbro and gabbronorite units alternate with lenticular strataboundintercalations of ultramafic (peridotite and pyroxenite) cumulates.A well-developed hybrid footwall zone of intermingled gabbroand pyroxenite underlies each ultramafic unit and demonstratesthe intrusive relationships of ultramafics into gabbroic cumulatemembers. The limited range of mg-number [100 ? Mg/(Mg+Fe)] of ferromagnesiansilicates indicates that the magmatic sequence covers a rathersmall spectrum in chemical fractionation and that the WingellinaHills intrusion represents the basal portion of a formerly largerlayered complex. The mg-number of olivine ranges from 89 to77, below which olivine is replaced by cumulus orthopyroxene.Clinopyroxene covers a wider mg-number range from 91 to 77 andis systematically enriched in MgO relative to coexisting orthopyroxeneand olivine. Anorthite content in plagioclase generally correlatespositively with mg-number changes of coexisting ferromagnesiansilicates. Interstitial plagioclase in clinopyroxenites containsexsolution lamellae of pure orthoclase. These antiperthitesare among the most calcic recorded, with plagioclase hosts betweenAn60 and An80. Bulk antiperthite compositions range around An65–Ab15–Or20and straddle a high-temperature (Or20) solvus in the plagioclasetriangle. The extent of former solid solution between calcicplagioclase and orthoclase indicates crystallization and coolingof the cumulates under moderate pressure and anhydrous conditions. Cryptic mg-number variations show that the intrusion experiencedweak iron enrichment with stratigraphic height. Normal fractionationis confined to the gabbroic members of the sequence, whereasultramafic intercalations are associated with sharp chemicalreversals toward more refractory mineral compositions. Reversalsof mg-number are considerably displaced into the underlyinggabbroic units by up to 50 m relative to the basis of ultramaficintercalations, which indicates extensive postcumulus infiltrationmetasomatism following the emplacement of fresh magma. The trivalentoxides in clinopyroxene have retained their pristine stratigraphicvariation patterns through later metasomatic events and stillcoincide with the cumulus layering. Macroscopic and cryptic layering in the Wingellina Hills intrusionare consistent with a continuously fractionating magma chamberwhose differentiation path was repeatedly reset by periodicinfluxes of primitive parent melt. Ultramafic and gabbroic cumulatemembers can be derived from a single olivine-saturated parentmelt by sequential separation of olivine, olivine-clinopyroxene,and finally olivine/orthopyroxene-clinopyroxene-plagioclase.A series of orthopyroxene-rich cumulates in the mixing zonesof the two melts crystallized from hybrids of the most primitiveand most evolved end-member compositions. Liquidus temperatures calculated for the resident and replenishingmelt components yield 1250 and 1350?C, respectively. As a resultof this temperature difference, fresh influxes of hot parentliquid crystallized rapidly under strongly undercooled conditionsas they ponded on, and quenched against,the chamber floor. Rapidcooling caused a temporary acceleration of the crystallizationfront and formation of impure cumulates with high trapped meltproportions, which resulted in a close coincidence of orthocumulateunits with stratigraphic levels of primitive melt addition.Grain sizes in orthocumulates vary with the cooling rate andpass through a maximum as the degree of undercooling increases.High cooling rates also influenced the composition of some cumulusphases. Clinopyroxenes from ultramafics in the mixing zonesare enriched in iron and aluminium (despite a more primitiveparent melt) and fall outside the fractionation path, especiallyif the batch of new hot magma was small compared with the poolof cooler resident liquid. Aluminous cumulus spinel is partof a metastable crystallization sequence and only crystallizedin the most magnesian ultramafics after episodes of intraplutonicquenching.  相似文献   

14.
《International Geology Review》2012,54(11):1395-1412
The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al2O3, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial 87Sr/86Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207Pb/204Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.  相似文献   

15.
Carbonate and calcareous-quartzite of Miri-Buxa Group in the Siang Window of Eastern Himalaya intruded by mafic rocks of gabbroic affinity. These intrusive rocks are low-Ti tholeiites (Ti/Y = 379−478; Nb/La = 0.99−1.88) and characterized by enriched LILE-LREE, depleted in HFSE with minor REE fractionation [(La/Yb)N = 2.72−3.35)]. Geochemical behaviour of the incompatible trace elements with the rare earth elements abundances indicates their cogenetic nature and their emplacement in a continental rift tectonic environment. The liquidus olivine temperature of these mafic rocks ranges from 1262°C to 1380°C showing a gentle decrease of [Mg] with a steep increase of [Fe]. These charters thus imply that the rocks are either related to the extent of common source or fractionational crystallization of plagioclase and clinopyroxene from a single batch parental magma. Petrogenetic modeling of [Mg]-[Fe] and REE indicates that these mafic intrusives probably derived from a mantle source similar to komatiitic composition at moderate to high degree (8%–20%) of partial melting.  相似文献   

16.
 Garnet–hornblende–plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An εNd( 530 ) value of +5.6 for the gneisses as well as initial 87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet–hornblende–plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U–Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532±30 Ma, similar to previously dated (mostly calc-alkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian–early Palaeozoic off the northern continental margin of Gondwana. Received: 26 April 1996 / Accepted: 2 August 1996  相似文献   

17.
《International Geology Review》2012,54(11):1401-1417
The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO2 (38–52 wt.%) and high Mg# (～48–70), Ni (100–470 ppm), and Cr (180–1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus minerals. Their Nb/Y and Zr/TiO2 ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ? Nd(t) = 5.3–8.5 and 147Sm/144Nd = 0.18–0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian–early Carboniferous arc developed along the west margin of Gondwana.  相似文献   

18.
Geological and geochemical data from the Coolac Serpentinite Belt, North Mooney Complex and Honeysuckle Beds in southern New South Wales indicate that the association represents a partially dismembered ophiolite suite. All the ideal ophiolite units are present, although the sheeted dyke unit is apparently of limited extent. Major and trace element (including rare‐earth) results, relationships of mafic volcanics and dolerite to massive and layered gabbroic and ultramafic intrusives, the occurrence of tectonised harzburgite with podiform chromitite, and the nature of pyritic sulphide occurrences, are consistent with a marginal sea or back‐arc environment for the generation of the ophiolite suite.  相似文献   

19.
We report experimental results and whole-rock trace-elementcharacteristics of a corundum-bearing mafic rock from the Horomanperidotite complex, Japan. Coronitic textures around corundumin the sample suggest that corundum was not stable in maficrock compositions during the late-stage P–T conditionsrecorded in the complex (P < 1 GPa, T < 800°C). Basedon the experimental results, corundum is stable in aluminousmafic compositions at pressures of 2–3 GPa under dry conditions,suggesting that the corundum-bearing mineral assemblages developedunder upper-mantle conditions, probably within the surroundingperidotite. Variations in the trace-element compositions ofthe corundum-bearing mafic rock and related rocks can be controlledby modal variations of plagioclase, clinopyroxene and olivine,suggesting that they formed as gabbroic rocks at low-pressureconditions, and that the corundum-bearing mafic rock was derivedfrom a plagioclase-rich protolith. A complex P–T trajectory,involving metamorphism of the plagioclase-rich protolith ata pressure higher than that at which it was first formed, isneeded to explain the origin of the corundum-bearing mafic rocks.They show no evidence for partial melting after their formationas low-pressure cumulates. The Horoman complex is an exampleof a large peridotite body containing possible remnants of subductedoceanic lithosphere still retaining their original geochemicalsignatures without chemical modification during subduction andexhumation. KEY WORDS: Horoman; mafic rock; corundum; experiment; P–T history; recycling  相似文献   

20.
We present mineralogical, petrological and geochemical datato constrain the origin of the Harzburg mafic–ultramaficintrusion. The intrusion is composed mainly of mafic rocks rangingfrom gabbronorite to quartz diorite. Ultramafic rocks are veryrare in surface outcrops. Dunite is observed only in deepersections of the Flora I drill core. Microgranitic (fine-grainedquartz-feldspathic) veins found in the mafic and ultramaficrocks result from contamination of the ultramafic magmas bycrustal melts. In ultramafic and mafic compositions cumulatetextures are widespread and filter pressing phenomena are obvious.The order of crystallization is olivine pargasite, phlogopite,spinel plagioclase, orthopyroxene plagioclase, clinopyroxene.Hydrous minerals such as phlogopite and pargasite are essentialconstituents of the ultramafic cumulates. The most primitiveolivine composition is Fo89·5 with 0·4 wt % NiO,which indicates that the olivine may have been in equilibriumwith primitive mantle melts. Coexisting melt compositions estimatedfrom this olivine have mg-number = 71. The chemical varietyof the rocks constituting the intrusion and the mg-number ofthe most primitive melt allow an estimation of the approximatecomposition of the mantle-derived primary magma. The geochemicalcharacteristics of the estimated magma are similar to thoseof an island-arc tholeiite, characterized by low TiO2 and alkalisand high Al2O3. Geochemical and Pb, Sr and Nd isotope data demonstratethat even the most primitive rocks have assimilated crustalmaterial. The decoupling of Sr from Nd in some samples demonstratesthe influence of a fluid that transported radiogenic Sr. Leadof crustal origin from two isotopically distinct reservoirsdominates the Pb of all samples. The ultramafic rocks and thecumulates best reflect the initial isotopic and geochemicalsignature of the parent magma. Magma that crystallized in theupper part of the chamber was more strongly affected by assimilatedmaterial. Petrographic, geochemical and isotope evidence demonstratesthat during a late stage of crystallization, hybrid rocks formedthrough the mechanical mixing of early cumulates and melts withstrong crustal contamination from the upper levels of the magmachamber. KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination  相似文献   

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