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1.
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   

2.
Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic–dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P2O5 anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at ～830 °C/～5 kbar. The crystallization conditions of the quartz diorites were estimated at ～570 °C/～2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust.  相似文献   

3.
The Mount Stuart batholith is a Late Cretaceous calc-alkaline pluton composed of rocks ranging in composition from two-pyroxene gabbro to granite. Quartz diorite is most abundant. This batholith may represent the plutonic counterpart of the high-alumina basalt association. A petrogenetic model is developed in which this intrusive series evolved from one batch of magnesian high-alumina basalt, represented by the oldest intrusive phase, by successive crystal fractionation of ascending residual magma. However, the possibility that this intrusive suite originated from an andésite (quartz diorite) parent by fractionation cannot be excluded.Computer modeling of this intrusive sequence provides a quantitative evaluation of the sequential change of magma composition. These calculations clearly indicate that the igneous suite is consanguineous, and that subtraction of early-formed crystals from the oldest rock is capable of reproducing the entire magma series with a remainder of 2–3% granitic liquid. This model requires that large amounts of gabbroic cumulate remain hidden at depth- an amount equal to approximately 8–10X the volume of the exposed batholith. Mass balances between the amounts of cumulate and residual liquid calculated compare favorably with the observed amounts of intermediate rocks exposed in the batholith, but not with the mafic rocks.Mafic magmas probably fractionated at depth by crystal settling, whereas younger quartz diorite and more granitic magmas underwent inward crystallization producing gradationally zoned plutons exposed at present erosional levels.  相似文献   

4.
The Upper Proterozoic ophiolite complex of Bou Azzer, Morocco, includes ultramafic rocks, cumulate gabbros, sheeted dykes, pillow lavas and diorite-quartz diorite intrusions and an overlying volcano-sedimentary sequence. The gabbroic cumulates, basaltic flows and dykes have compositions similar to recent ocean-floor rocks (N- and/or T-type). Among other features, they have comparable light REE-depleted patterns and relations of Ti-Zr and La-Nb. Although fractional crystallization played an important role in the evolution of these rocks, the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and/or by a dynamic partial melting process. Diorites, quartz diorites and the volcanic rocks of the overlying sequence are calc-alkaline, genetically unrelated to the tholeiitic suite and indicative of an island arc setting. A possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc.  相似文献   

5.
The Wadi Dabr intrusive complex, west of Mersa-Alam, Eastern Desert, Egypt ranges in composition from gabbro to diorite, quartz diorite and tonalite. The gabbroic rocks include pyroxene-horn blend e gabbro, hornblende gabbro, quartz-hornblende gabbro, metagabbro and amphibolite. Mineral chemistry data for the gabbroic rocks indicate that the composition of clinopyroxenes ranges from diopside to augite and the corresponding magma is equivalent to a volcanic-arc basalt. Plagioclase cores range from An75 to An34 for the gabbroic varieties, except for the metagabbro which has An 11–18. The brown amphiboles are primary phases and classified as calcic amphiboles, which range from tschermakitic hornblende to magnesiohornblende. Green hornblende and actinolite are secondary phases. Hornblende barometry and hornblende-plagioclase themometry for the gabbroic rocks estimate crystallisation conditions of 2–5 kb and 885–716°C.The intrusive rocks cover an extensive silica range (47.86–72.54 wt%) and do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO2, Al2O3, FeO*, MgP, CaO, P2O5, Cr, Ni, V, Sr, Zr and Y). Most of these elements exhibit two geochemical trends suggesting two magma sources.The gabbroic rocks are relatively enriched in large ion lithophile elements (K, Rb, Sr and Ba) and depleted in high field strength elements (Nb, Zr, Ti and Y) which suggest subduction-related magma. Rare earth element (REE) data demonstrate that the gabbroic rocks have a slight enrichment of light REE [(La/Yb)N=2.67−3.91] and depletion of heavy REE ((Tb/Yb)N=1.42−1.47], which suggest the parent magma was of relatively primitive mantle source.The diorites and tonalites are clearly calc-alkaline and have negative anomalies of Nb, Zr, and Y which also suggest subduction-related magma. They are related to continental trondhjemites in terms of Rb---Sr, K---Na---Ca, and to volcanic-arc granites in terms of Rb---and Nb---Y.The Wadi Dabr intrusive complex is analogous to intrusions emplaced in immature ensimatic island-arcs and represents a mixture of mantle (gabbroic rocks) and crustal fusion products (diorites and tonalites) modified by fractional processes.  相似文献   

6.
Rocks of the Moruya Batholith range from gabbros and gabbroic diorites through quartz diorites and tonalites to granodiorites and rare adamellites. The gabbros and gabbroic diorites appear as small, early bodies intruded and enclosed by quartz diorites and tonalites. These early gabbroids are petrographically and chemically distinct from the granitoids. The latter occur as a meridionally‐oriented sequence of nine separate plutons. Mafic xenoliths are most abundant in the quartz diorites and tonalites; they are petrographically similar to their host granitoids and are chemically a more mafic extension of the variation in granitoid compositions. The various granitoid bodies are considered to be derived from similar source rocks, with the xenoliths representing modified material relict from partial melting of that source.

Comparison of chemical data from the Moruya granitoids with those of the I‐types of the Jindabyne Suite in the Kosciusko Batholith, shows that the potassium content is indistinguishable in the two suites from each side of the Moruya‐Kosciusko Province, although elsewhere it has been shown to vary systematically across an orogenic belt. The most outstanding difference is the higher Na and Ti and lower Ca in the Moruya Batholith compared with those in Kosciusko Batholith I‐type granitoids.  相似文献   

7.
The Late Cretaceous Badjal intrusive suite at the Far East of Russia includes a spectrum of rocks having emplaced successively in four phases, from (i) diorite and quartz diorite to (ii) granodiorite, (iii) granite and (iv) high-silica granite, various facies being distinguished within the phases. The generation of these rocks took place after the collision of a number of island arcs and a terrane now locally preserved in Sikhote-Alin region with the Asian continent, that had happened in the end of Early to the beginning of Late Cretaceous. The massifs intrude in Early Mesozoic sedimentary and volcanic–sedimentary rocks topping the basement, and in comagmatic volcanic rocks. Chemically, the granitoid rocks have high-K calc-alkaline character and form continuous and regular trends of most of major oxides and trace elements with the SiO2 contents ranging from 55 to 77 wt.%, that favors the concept of crystal fractionation. Major oxides and REE have a break at 70 wt.% SiO2 pointing to a change of precipitating mineral assemblage. Such change is also documented by the negative Eu anomaly. We believe that the fractionation of mafic minerals had to take place at the first stage of fractionation, whereas plagioclase and possibly biotite began precipitating later, as supposed by abrupt decrease of Ba. This hypothesis is in accordance with the mass balance calculations. Diorite magma compositionally similar to the first intrusive phase rocks should be taken for parental for the entire Badjal suite. Small volume of these oldest rocks makes us suggest that the parental magma was a mixture of silicic liquid and restite mineral phases that would result from the partial melting of a heterogeneous metapelite–basaltic or metapelite–amphibolitic protolith. Almost total absence of basalt and gabbro of close age in the area makes a suggestion of crustal silicic contamination of a mantle-produced basic magma unlikely. After the melting, the differentiation of the derived magmatic mixture during ascent and the separation of mineral phases acquired from the protolith (restite phases) and of newly formed ones are believed to produce the entire range of rocks of the Badjal intrusive suite, provided the progressive succession of emplacement from less to more silicic species.  相似文献   

8.
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   

9.
The Wadi Nesryin gabbroic intrusion is part of the Neoproterozoic Pan-African basement cropping out in southern Western Sinai of Egypt. The intrusion comprises hornblende gabbro, pyroxene–hornblende gabbro, diorite and appinitic varieties. It exhibits chilled margins against the older rocks represented by fine-grained gabbro and dolerite and belongs to what is known throughout Egypt as the “younger gabbro suite”. Mineralogy, mineral chemistry and whole rock geochemistry indicate that these rocks were derived from tholeiitic magmas with minor calc-alkaline affinity. They have chemical signatures of subduction related arc rocks formed at an active convergent plate margin. They were formed by 15–30% of partial melting of a garnet lherzolite and to a minor extent of spinel-garnet lherzolite sources, modified by fluids related to a subducting slab. Pressure estimates using the amphibole geobarometer indicate that the gabbroic rocks crystallized at pressures between 2.8 and 5.6 kbar (average?=?4.3 kbar). Diorites record lower formation pressures between 1.8 and 3.7 kbar (average?=?3.0 kbar). The temperature estimates calculated by several geothermometers yielded crystallization temperatures ranging from 674°C to 961°C, with an average of about 817°C. The whole rock Rb–Sr isochron age of the Nesryin gabbroic intrusion is 617?±?19 Ma with initial 87Sr/86Sr?=?0.70322?±?0.00004. This age indicates that the mafic–ultramafic plutons in the Pan-African belt in southern Sinai belong to the Egyptian younger gabbros and not to the older metagabbro–diorite complexes or ophiolitic suites. The rocks have low 87Sr/86Sr initial ratios ranging from 0.703141 to 0.703338 and negative ? Sr ranging from ?6.34 to ?9.14. The initial 143Nd/144Nd ratios range from 0.511944 to 0.512145 with positive and high ? Nd values (1.93 to 5.86) reflecting a mantle contribution in their petrogenesis.  相似文献   

10.
The Bear Mountain igneous complex, Klamath Mountains, California,can be divided into distinct lithologic suites (order accordingto apparent relative age): (1) satellitic masses of clinopyroxene-richultramafic and gabbroic rocks with subordinate dunite and hornblende-plagioclasepegmatoid; (2) two-pyroxene-biotite diorite and monzodiorite;(3) heterogeneous hornblende-rich rocks varying from gabbroto diorite; (4) leucocratic rocks, chiefly consisting of biotitetonalite and granodiorite; and (5) late dikes (mafic to felsic).Elongate masses of unit (1) flank a composite pluton consistingof units (2–4), while the late dikes (unit 5) intrudethe adjacent country rocks. The rocks of the complex invadedan ophiolite allochthon during the Late Jurassic Nevadan orogeny,and well-defined contact aureoles surround the complex. Lowergreenschist facies rocks, chiefly metabasalt, impure siliceousmetasedimentary rocks, and serpentinized peridotite, have beendynamothermally metamorphosed to mineral assemblages indicativeof hornblende-hornfels facies and locally pyroxene-hornfelsfacies. The emplacement of the igneous complex was chiefly byforcible shouldering aside, although local tectonic featuressuch as faults in the ophiolite allochthon were instrumentalin the emplacement history. The ultramafic and gabbroic rocks are interpreted as crystalcumulates of a fractionated basaltic magma. Mineral compositionsand whole-rock chemical characteristics of the proposed cumulatessuggest that the Mg/Fe ratio of the parental basaltic liquidwas high. The activity of silica was low, while water vaporpressure apparently increased through time until it was moderatelyhigh during the late magmatic stage. These cumulates were subsequentlyremobilized during lateral tectonic compression and emplacedhigher in the crust as hot, semisolid aggregates. A diverse array of data, including pyroxene compositions, major-,minor-, and rare-earth-element abundances and field relations,suggest that the two-pyroxene-biotite diorite/monzodiorite unitwas consanguineous with the clinopyroxene-rich ultramafic andgabbroic rocks. The diorite/monzodiorite unit, therefore, isan intermediate differentiate of an early primitive basalt.Furthermore, major-, trace, and rare-earth-element data characteristicof the diorite/monzodionte unit indicate strong similaritiesto low-Si andesite and clearly suggest a calc-alkaline affinity. Age relations indicate that the hornblende-rich and leucocraticunits are younger and represent the intrusion of other magmasinto the same igneous locus. Petrographic and geochemical datafrom the hornblende-rich unit suggest recrystallization fromhydrous magmas similar in composition to high-Al basalt andbasaltic andesite. The leucocratic suite, consisting chieflyof calc-alkaline tonalitic rocks, is similar to other quartz-richfelsic rocks widespread throughout the Klamath Mountains-westernSierra Nevada. The available petrographic and geochemical dataare consistent with formation of these rocks by either fractionalcrystallization of a wet basaltic magma or partial melting ofamphibolite or eclogite. The Bear Mountain igneous complex is an example of a diversebut distinctive association of ultrabasic to silicic rocks whichcharacterize numerous plutonic complexes in the Klamath Mountains-westernSierra Nevada. These intrusive complexes invade older ensimaticrocks and appear to define the roots of a complex, Middle toLate Jurassic calc-alkaline magmatic arc. The ultramafic andgabbroic rocks characteristic of this plutonic association aresimilar to Alaskan-type complexes but differ in detail. Moresignificantly, these rocks are important clues to the compositionof early magmas as well as the complex processes operative inreservoirs that form the core of calc-alkaline magmatic centers.  相似文献   

11.
The south Ardestan plutonic rocks constitute major outcrops in the central part of Iran’s Cenozoic magmatic belt and encompass a wide compositional spectrum from gabbro to granodiorite. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating of zircon three granodiorites yielded ages of 24.6 ± 0.1, 24.6 ± 0.1, and 24.5 ± 0.1 Ma. For tonalitic rocks, internal Rb–Sr isochron ages (biotite, feldspars) indicate cooling ages of 20.4 ± 0.1, 20.5 ± 0.1, and 22.3 ± 0.1 Ma, which are slightly younger than the zircons’ ages. The limited variations in their Sr–Nd isotope ratios indicate derivation from an asthenospheric mantle source. A geodynamic model is presented in which late Oligocene–Miocene rollback of the Neotethyan subducting slab triggered asthenospheric upwelling and partial melting in the south Ardestan. These melts were subsequently modified through fractional crystallization and minor crustal contamination en-route to the surface. Plagioclase + orthopyroxene-dominated fractional crystallization accounts for differentiation of gabbro to gabbroic diorite, whereas fractionation of clinopyroxene, titanomagnetite, and orthopyroxene led to differentiation of gabbroic diorite to diorite. Amphibole fractionation at deeper levels led to the development of tonalites.  相似文献   

12.
The Lyngen gabbro (LG), defining the major part of the Lyngen magmatic complex, is characterised by layered gabbros of N-MORB affinity (western suite) and layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiite) to boninitic affinity (eastern suite). The boundary between the eastern and western suites is generally defined by a large-scale ductile shear zone of suboceanic origin, the Rypdalen shear zone (RSZ). Tonalites occur within the RSZ and in the eastern suite of the LG. Variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups. Tonalite intrusion (the Vakkas pluton) up to 5 km2 large occur in the eastern suite of the LG, and are characterised by high Y contents (average 26 ppm) and high K2O/Rb ratios (average 0.062) compared to tonalites on the RSZ. The Vakkas pluton has lightly concave REE (rare earth element) patterns with negative Eu-anomalies, and positive ND-values (+3.7 to +3.9). Geochemical modelling based on the REE and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma. Tonalites related to the RSZ form irregular veins and dikes that net vein the shear zone. They are characterised by low Y contents (average 6 ppm), low K2O/Rb ratios (average 0.025), and highly variable contents of Na2O, K2O, Sr and Ba, compared to the Vakkas pluton. Tonalites related to the RSZ show substantial variation in the content of the LREEs. They possess low abundances of the HREEs, and absence of, or slightly positive Eu-anomalies. The tonalites have highly variable ND-values (−0.6 to −9.4), probably resulting from enrichment of Nd from an external source. Geochemical modelling suggests that the LREE-rich tonalites formed by H2O-rich partial melting of differentiated products from the eastern suite of the LG. The presence of B in the fluid phase is suggested by the presence of tourmaline-bearing tonalite pegmatites. Thus, the anatectic tonalites of this group could have been formed by water-excess melting of a variety of gabbroic cumulates of the LG. In the LG, LREE-depleted tonalites (ND-values +5.1) also occur, and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the LG.  相似文献   

13.
《International Geology Review》2012,54(15):1941-1958
The Qiarbahete complex in NW China consists of gabbroic diorite, granodiorite, and late-stage quartz diorite porphyry veins. Zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses show that the gabbroic diorite and granodiorite formed at 368 ± 5.2 Ma and 354 ± 4.1 Ma, respectively, indicating that the complex was emplaced in the Late Devonian–Early Carboniferous. The gabbroic diorites, characteristic of Sanukitoids, exhibit high Mg# (62 average), MgO (6.84% average), Cr (195 ppm average), and Ni (61.4 ppm average) contents. The rocks show moderately fractionated rare earth element (REEs) patterns and weak negative Eu anomalies (δEu: 0.83–0.89), enrichment of large ion lithophile elements (LILEs), and depletion of high field strength elements (HFSEs), with low ?Nd(t) values (1.46–1.73). The gabbroic diorites originated from partial melting of a hydrous mantle wedge followed by assimilation of crust during ascent. The granodiorites show a geochemical affinity with adakitic rocks, e.g. SiO2 (64.95–67.87%) > 56%, Al2O3 (15.88–16.56%) > 15%, MgO (1.79–2.31%) < 3%, Sr (315–375 ppm) > 300 ppm, and Yb (1.84–2.06 ppm). They are enriched in light rare earth elements (LREEs) and LILEs and depleted in HFSEs, with weak negative Eu anomalies (δEu: 0.78–0.87). The granodiorites were mainly derived by the partial melting of a subducted oceanic slab, followed by subsequent melt–mantle interaction and crustal rocks contamination. All these indicate that the Qiarbahete complex was emplaced in a continental arc setting attending the southward subduction of the Junggar Ocean during the Late Devonian–early Carboniferous, generating the lateral accretion of continental crust in NW Tianshan.  相似文献   

14.
Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.  相似文献   

15.
Quartz diorites represent the earliest (ca. 540 Ma) and most primitive plutonic rocks in the Pan African Damara belt and they pre-date the main phase of high-T regional metamorphism. Two suites of synorogenic quartz diorites are unusual among Damaran intrusive rocks in their elemental and isotopic features. Comparison of the diorite compositions with melts from amphibolite-dehydration melting experiments points to a garnet-bearing meta-tholeiite, probably enriched in K2O, as a likely source rock. Partial melting processes generated mafic (ca. 50 wt% SiO2) quartz diorites in the deep crust at temperatures of between 1,000 and 1,100 °C, based on comparison with experimental results and similar temperature estimates based on P2O5 solubility in mafic rocks. Subsequently, the quartz diorites evolved by multistage, polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses. Although their chemical characteristics (high LILE, low HFSE) resemble those of other quartz diorites with calc-alkaline affinities, they differ in their enriched Sr (initial 87Sr/86Sr: 0.70943-0.71285), Nd (initial ) Nd: -9.1 to -15.2 ) and O ('18O: 6.8-8.1‰) isotope compositions. Neodymium model ages (TDM) that range from 1.7 to 2.2 Ga and large variation in 207Pb/204Pb relative to 206Pb/204Pb indicates involvement of ancient crustal material. Lead (206Pb/204Pb: 17.08-17.23, 207Pb/204Pb: 15.53-15.62, 208Pb/204Pb: 37.71-38.16) isotope compositions are strongly retarded, indicating that the source underwent a pre-Pan-African U/Pb fractionation and U depletion. It is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust. This contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during Pan-African times with characteristics modified by subduction of oceanic crust and sedimentary rocks.  相似文献   

16.
The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.

The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al2O3 (>>16%), low TiO2 (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite.  相似文献   


17.
Gabbroic intrusions of the El-Aradiya area are a part of the Neoproterozoic basement cropping out in the central Eastern Desert of Egypt. They are composed mainly of gabbroic cumulates (diopside-plagioclase cumulate and plagioclase-augite cumulate) and fine-grained noncumulate gabbro. Mineral chemistry data indicate that the plagioclase core compositions of the gabbroic cumulates range between An90 and An60, whereas fine-grained noncumulate gabbro plagioclase core compositions are An61−56 and rim compositions are An54−42. The clinopyroxenes are diopside and augite in the gabbroic cumulate, and augite in the fine-grained noncumulate gabbro. Chemical re-equilibration between pyroxenes of gabbroic cumulates vary from 1150-900°C and for fine-grained noncumulate gabbro range from 1200-1100°C. The amphiboles are calcic, varying from tschermakite and tschermakitic hornblende, and Mg-hornblende in the gabbroic cumulate and only Mg-hornblende in the fine-grained noncumulate gabbro. They indicate an island-arc tholeiitic setting for gabbroic intrusions of the El-Aradiya area. Major and trace element data suggest arc tholeiite characters, a comagmatic suite and subduction-related magma with enrichment of LILE and depletion in HFSE relative to MORB. The estimated parent magma is similar to tholeiitic Aleutian arc primary magma. The gabbroic intrusions are analogous to intrusions emplaced in an immature island-arc setting in which the oceanic crust was thin.  相似文献   

18.
The Pleasant Bay layered gabbro–diorite intrusion, locatedon the coast of Maine between Bar Harbor and Machias, is roughlyoval in plan, measuring 12 km by 20 km. Gravity data, contactrelations, and internal layering suggest that it is basinformin structure with a maximum thickness of {small tilde}3 km.Its roof and upper parts have been lost through erosion. Whereit is in contact with underlying granite, the base of the intrusiontypically consists of strongly chilled gabbro with convex-downwardlobate forms, suggesting that the granite was incompletely solidifiedwhen the gabbro was emplaced. Roughly 90% of the exposed rocksare weakly layered gabbro and mafic diorite, both of which varywidely in grain-size and texture. Layers and lenses of medium-grainedleucocratic diorite to granodiorite are widely intercalatedwith the chilled mafic rocks and commonly contain partly digestedmafic inclusions; they also commonly contain zones of pillow-likebodies of gabbro chilled on all margins. The dioritic rocksare consistently topped by gabbroic layers with chilled lobatebases and commonly appear to feed granitic pipes and diapirsinto overlying gabbro. Much of the intrusion can be subdividedinto hundreds of macrorhythmic units (from 1 to 100 m thick)consisting of basally chilled gabbro that grades upward to dioriteor highly evolved leucocratic silicic cumulates. Basaltic dikesare abundant both in the underlying granite and in the layeredgabbro–diorites; they have appropriate compositions tobe feeders for chilled gabbroic layers in the Pleasant Bay intrusion. The layered rocks of the Pleasant Bay intrusion record hundredsof basaltic injections into a chamber with resident silicicmagma. Small injections produced chilled gabbroic layers andpillows within silicic cumulates. Larger infusions of basalticmagma produced temporary compositional stratification and episodesof double-diffusive convection within the chamber. Althoughfractional crystallization produced compositional variationin much of the gabbro, units that grade from chilled gabbroat the base to highly silicic cumulates at the top provide cumulaterecords of magma stratification and hybridization along a double-diffusiveinterface between basaltic and silicic magmas. The intrusionprovides a superb plutonic record of events that have oftenbeen inferred for silicic eruptive centers. Mafic–siliciclayered intrusions comparable with the Pleasant Bay are morewidespread than has generally been appreciated.  相似文献   

19.
The Nabar pluton with the age of Oligo-Miocene located northwest of Isfahan, the Urumieh-Dokhtar magmatic belt, is composed of gabbro, gabbro diorite, diorite, quartz diorite, tonalite, and quartz monzonite. These rocks contain plagioclase, quartz, alkali-feldspar, magnesiohornblende, actinolite, tremolite-hornblende, actinolite-hornblende, anthophyllite, biotite, and Na-poor pyroxene. Application of the Al-in-hornblende barometry indicates pressures of 2–2.15 kbar, whereas the clinopyroxene barometry shows a pressure of 5 kbar. The temperature (i.e., 750–800°C) is estimated using the amphibole-clinopyroxene thermometry in a dioritic sample. Magmatic water content was greater than 10% at the time of formation of dioritic rocks in the Nabar pluton. Based on chemistry of mafic minerals and geochemical data, the Nabar plutonic complex comprises medium-K, calc-alkaline, and I-type granitoid. The rocks are characterized by enrichment of lithophile elements (LILEs) and depletion of high-field-strength elements (HFSEs). The Nabar rocks have weak concave-upward rare earth element (REE) patterns, suggesting that amphibole played a significant role in their generation during magma segregation. Low (Al2O3/(FeO + MgO + TiO2) and (Na2O + K2O)/(FeO + MgO + TiO2) ratios, and the patterns of trace and rare earth elements suggest that these rocks formed along a destructive plate margin and were derived from a lower crustal source. The magma probably formed by partial melting of lower crustal protoliths (amphibolites). Lower crust contamination with magma derived from partial melting of the upper mantle has an important role in the formation of this intrusive body, and a fractional crystallization of melts in higher crustal levels generated this spectrum of rock types. Mantle-derived gabbroic magmas emplaced into the lower crust are the most likely heat sources for partial melting.  相似文献   

20.
The Tichka Massif is one of many late-Paleozoic massifs emplaced in northwestern Africa. It consists of granitic pods surrounded by a complex assemblage of dioritic and gabbroic rocks. Previous workers have suggested an independent origin for the granitic and gabbroic rocks and this study evaluates this suggestion using REE data.The Ce/Yb ratio versus Ce plot for the gabbro shows a large Ce/Yb variation for small changes in Ce, whereas the granites show a linear increase of Ce/Yb with Ce. In the granites, the samples with the highest Ce/Yb ratio have the lowest SiO2, whereas in the gabbros the Ce/Yb ratio is lowest in the low SiO2 rocks. The diorites show a tightly clustered pattern.Based on the REE data the granite-diorite-gabbro sequence cannot be related by fractional crystallization. Partial melting of a garnet bearing mantle is consistent with the gabbroic trends. Partial melting of a crustal source with small amounts of apatite and abundant hornblende can produce the granitic rocks.  相似文献   

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