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1.
In the Segura area, Variscan S-type granites, aplite veins and lepidolite-subtype granitic aplite-pegmatite veins intruded the Cambrian schist-metagraywacke complex. The granites are syn D3. Aplite veins also intruded the granites. Two-mica granite and muscovite granite have similar ages of 311.0 ± 0.5 Ma and 312.9 ± 2.0 Ma but are not genetically related, as indicated by their geochemical characteristics and (87Sr/86Sr)311 values. They correspond to distinct pulses of magma derived by partial melting of heterogeneous metapelitic rocks. Major and trace elements suggest fractionation trends for: (a) muscovite granite and aplite veins; (b) two-mica granite and lepidolite-subtype aplite-pegmatite veins, but with a gap in most of these trends. Least square analysis for major elements, and modeling of trace elements, indicate that the aplite veins were derived from the muscovite granite magma by fractional crystallization of quartz, plagioclase, K-feldspar and ilmenite. This is supported by the similar (87Sr/86Sr)311 and δ18O values and the behavior of P2O5 in K-feldspar and albite. The decrease in (87Sr/86Sr)311 and strong increase (1.6‰) in δ18O from two-mica granite to lepidolite-subtype aplite-pegmatite veins, and the behaviors of Ca, Mn and F of hydroxylapatite indicate that these veins are not related to the two-mica granite.  相似文献   

2.
At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C.The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ18O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ18OH2O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite.The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δDH2O values for the metamorphic fluid decrease from ?5‰ in the glaucophane zone to an average of about ?70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend.Theδ18O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism.Detailed data on 20 marble units show systematic variations of δ18O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ18O gradients at the margins and large δ18O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ18O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates.Almost all the δ13C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ13C values of most marble units show a general correlation with δ18O values.The CO2H2O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ13C values of the CO2 range from ?8.0 to 3.6‰ and indicate that at some localities CO2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.  相似文献   

3.
Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI?=?molecular Al/[Na?+?K]) of the dykes is 1.20 (n?=?29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171?±?4.4 Ma, with variable initial 87Sr/86Sr ratio (0.7075?±?0.0032), and have an average ε Nd of ? 12.0. Quartz phenocrysts have δ18O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ18O values of quartz and aegirine (average Δquartz?aegirine = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε Nd (< ? 10) and magma δ18O values averaging 8.6‰ (assuming Δquartz?magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ18O values and much less negative ε Nd (average ??3.1, n?=?3). The country rock has similar δ18O values to the granite dykes (average 8.0‰, n?=?108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ18O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization.  相似文献   

4.
The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average δ18O value of 7.4±0.6‰ (all values±1σ). Four Barcroft specimens average εNd=?3.6±1.8, 87Sr/86Sr=0.707±0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite–aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average δ18O of 8.6±0.5‰. Four McAfee-type samples average εNd=?7.8±1.7, 87Sr/86Sr=0.711±0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5–10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low δ18O in the sequence quartz>plagioclase>K-feldspar>>amphibole≥biotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in δ18O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk 18O/16O ratios of Barcroft border rocks (and constituent plagioclase+subsolidus tremolite–actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz–amphibole and six quartz–biotite pairs indicate apparent subsolidus temperatures averaging 519±49 °C. Quartz–plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515 °C. A quartz–biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511 °C, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz–plagioclase, quartz–amphibole, and quartz–biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion ~70 million years later by the McAfee Creek Granite had no discernable effect on δ18O values of Barcroft minerals and bulk rocks.  相似文献   

5.
Quartz-eye keratophyric tuffs at Aljustrel, S. Portugal, Iberian Pyrite Belt possess unusually high, uniform whole rock δ-values up to 18.1%. with a mean of 16.7 ± 0.7%.. Because the quartz eye cores have δ18O 12.0 to 13.5%. the felsic tuffs may have had an original whole rock oxygen isotope composition of 10 to 11%. The deduced enrichments of up to +8%. in keratophyric derivatives is attributed to isotope exchange with abundant marine water under temperatures which diminished to ≤ 100°C, during thermally driven convective cooling. The isotopic uniformity requires elevated permeability of ～ 10?8 cm2, such that the second critical Rayleigh number was exceeded, and drifting, non steady state convection cells dominated, with relatively smooth thermal structures.In mineralised counterparts of the tuffs beneath the Feitais-Estacao Zn-Pb-Cu massive sulphide orebodies, whole rock δ-values are ~12%. and quartz (13.3–15.4%.)-chlorite (3.2–6.3%.) fractionations correspond to temperatures of 220–270°C, and a calculated fluid δ18O of 1.4 to 5.7%. This local isotopic overprinting in vent domains of the regionally high 18O was induced by a stable, second stage convective regime imposed by drastic reduction of permeability accompanying capping of the geothermal discharge by hydrothermal cherts, and/or progressive spilitisation of the tuffs. At lower water/rock coupled with higher temperatures, the recirculating marine water underwent variable 18O enrichment up to 5.7%. by exchange with the high 18O tuffs. An upwards increase in δ18O quartz and δ-quartz reflects progressive cooling of the geothermal discharge from 15.4%. 270°C in the stockwork, through 18.3%. 220–240°C in the orebody, to 20.1%. 110–130°C in overlying hydrothermal cherts, probably induced by entrainment of ambient marine bottom water in sub-seafloor aquifers.  相似文献   

6.
Carbon and oxygen isotopic analyses of silicate and carbonate minerals indicate that isotopic compositions in metasediments of the Wallace Formation (Belt Supergroup) exposed northwest of the Idaho batholith have been affected by both prograde and retrograde fluid-rock interaction. Silicates retain isotopic fractionations that reflect equilibration at peak metamorphic temperatures. In contrast, calcite oxygen isotopic compositions range from δ18O(Calcite)=+2.3 to +18.6‰ SMOW (standard mean oceanic water) and indicate that some calcites have exchanged with low-δ18O meteorichydrothermal fluids. Values of Δ18O (Quartz-Calcite) as large as +15.5 clearly indicate that the isotopic depletion of these calcites postdates the peak of regional metamorphism. Carbon isotopic compositions of 18O-depleted calcites are not significantly shifted relative to δ13C values in undepleted calcites, suggesting that the retrograde fluid was carbon-poor. Petrographically, retrograde fluid-rock interaction is associated with the occurrence of fine-grained, highly-luminescent calcite overgrowths on less-luminescent, metamorphic calcites, slight to moderate argillic alteration, and pseudomorphing of scapolite porphyroblasts by fine-grained albite. Retrograde isotopic depletions may be related to shallow meteoric-hydrothermal fluid flow developed around the Idaho batholith after intrusion and rapid uplift of the terrane. Peak metamorphic isotopic compositions in the Wallace Formation reflect mineralogically heterogeneous protolith compositions and isotopic fractionation due to devolatilization and/or infiltration. Variability in oxygen isotopic compositions on the order of 4–6‰ within the same rock type can be attributed to the combined effects of inherited isotopic compositions and isotopic shifts resulting from prograde devolatilization. Isotopic and compositional heterogeneity on the scale of mm to m precludes generalization of isotopic gradients on a regional scale. The isotopic data presented here, and metamorphic fluid compositions determined in previous studies, are best reconciled with heterogeneous bulk compositions, dominantly channelized prograde and retrograde fluid flow, and locally low fluid-rock ratios.  相似文献   

7.
Oxygen isotope ratios, whole rock major and trace element compositions, and petrological characteristics of 52 samples from nine distinct igneous lithologies in the lower plate of the Whipple Mountain metamorphic core complex of south-eastern California indicate that both mylonitic and non-mylonitic lithologies underwent exchange with surface-derived meteoric waters. Broadly granodioritic lithologies are characterized by whole rock δ18O values that range from 10.6 to 2.6‰. Isotopic compositions of quartz and feldspar mineral separates indicate that quartz has largely retained original igneous compositions but that feldspar has undergone variable and often large 18O-depletions (up to 6.5‰). Over 4 km of structural relief is exposed in lower plate gneisses below the Whipple detachment fault including non-mylonitic lithologies at shallow structural levels above the mylonite front, and mylonitic gneisses at intermediate to deep levels below the mylonite front. Coupled δ18Oqtz - δ18OFsp systematics of non-mylonitic and mylonitic andesite to rhyolite dykes from shallow and intermediate structural levels of the lower plate document two episodes of hydrothermal alteration: a high-temperature (>c.600d?C) episode involving a metamorphic or magmatic fluid with δ18O values ～ 7‰ and a low-temperature (c.350d?C) episode involving low-δ18O meteoric fluids. All the dykes that document exchange with meteoric fluids are non-mylonitic. Coupled δ18OFsp systematics of non-mylonitic and mylonitic granodioritic gneisses from above and below the mylonite front also document low-temperature (c. 350d? C) exchange with meteoric fluids. The data indicate that infiltration of meteoric fluids occurred as lower plate lithologies were juxtaposed against the base of the faulted upper plate. High-angle normal faults in the upper plate served as the conduits for the downward circulation of surface-derived fluids. Meteoric fluids were able to penetrate across the detachment fault into the lower plate. Uplift rates coupled with independent cooling rates indicate that surface-derived fluids penetrated to a depth of c.4km and possibly as deep as c.8km. Penetration of surface-derived fluid into the ductile deformation regime is not required to explain the low δ18O values observed in lower plate lithologies of the Whipple Mountain metamorphic core complex.  相似文献   

8.
In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ18O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ18O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable 18O/16O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of 18O/16O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ18O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in 18O/l6O must be a result of exchange with magmatic or metamorphic H2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ18O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system.

LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H2O, aided by internal muscovite dehydration at ?8 kbar and ?750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (?30°) partial melting of the metapelites produced magmas near H2O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H2O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H2O. The released H2O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H2O was released and again re-used for a final episode of 18O/l6O exchange with the unmelted metamorphic lithologies. In addition to the direct l8O/16O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H2O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.  相似文献   

9.
Abstract Coexisting quartz, feldspar and biotite vary widely in their δ18O values and display a remarkable 18O / 16O disequilibrium relation; especially, a quartz–feldspar reversal (Δ 18OQuartz?feldspar<0) exsists in the Aral granite pluton of the Altay Mountains, northern Xinjiang. The 18O / 16O exchange reaction definitely occurred between granite and water. Initial δ18O values of the granite and exotic fluid are evaluated by the mass balance consideration. The conventional method of discrimination between various magma derivations simply with δ18O values of either whole rock or separate minerals is misleading and unreliable. Experiments carried out by the authors show that the 18O / 16O exchange reaction is not accompanied by what geologists describe as petrological and mineralogical alteration effect. This decoupling relation implies that exchange reaction occurs at a relatively high temperature during subsolidus–postmagmatic cooling of magmas. The exchange mechanism is mainly diffusion–controlled. It is demonstrated through quantitative modelling that the hydrothermal system associated with the Aral pluton is long–lived (0.8—6 Ma), with a relatively high fluid flow rate (3 x 10?14 mol. s?1) and high W/ R ratio (0.79-6.14). This means that an intense flow and convection may exist at the midcrustal level of orogenic magmatic arcs.  相似文献   

10.
Incipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew. Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200-310d? C (slightly higher in the Marloes-Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6-13 km. Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes-Musselwick Thrust Sheet are distinctly lighter (δ18Oveins=+14 to +18% and δ18Omatrix=+11 to +14%) than those from other thrust sheets (δ18O =+17 to +20% and +14 to +17%, respectively). We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large-scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.  相似文献   

11.
18O/16O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The 18O/16O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ 18O of quartz of 12 to 14‰ (SMOW) and δ 18O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in 18O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high 18O/16O ratios of granitic rocks of this district were discussed in relation to the 18O-depletion in metasediments. Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive 18O-depletion or 18O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.  相似文献   

12.
Analyses of 230 Franciscan rock and mineral samples, including the San Luis Obispo ophiolite, show that metamorphism produces no change in the δ18O of the graywackes (+11 to +14), but that igneous rocks become enriched in 18O by 2–6% and the cherts depleted by 5–10%. The shales are of two types, a high-18O type (+16 to +20) associated with chert and a low-18O type isotopically and mineralogically similar to the graywackes. The vein quartz (δ = + 15 to + 20) is invariably richer in 18O than the host rock quartz and in most of the rocks the δ18O of the clastic quartz is similar to the δ18O of the whole rock. Mineral assemblages are typically not in isotopic equilibrium. Although the δ18O values are very uniform (+13 to +16). the δ13C of vein aragonite and calcite is widely variable (0 to ? 14), implying that a major source of the carbon is oxidized organic material. The δD values of 83 igneous and sedimentary rocks are -45 to -80, exceptions are the Fe-rich minerals howieite and deerite, which have δD = ?100. All of these samples could have equilibrated with H2O having δD ≈ +10 to ?20 and δ18O ≈ ?3 to +8. assuming temperatures of 100–300°C. However, the serpentines (δD ≈ ?85 to ?110) and the vein minerals (δD = ?23 to ?55) are exceptions. The vein minerals are 10–20%, richer in deuterium than the adjacent wall rocks; they formed from a relatively D-rich metamorphic water, typically at lower temperatures than did their host rocks. The isotopic compositions of the other Franciscan rocks were affected by three distinct events: (1) hydrothermal alteration of the ophiolite complexes and volcanic rocks as a result of submarine igneous activity at a spreading center or in an island-arc environment; (2) low-temperature, high-pressure regional metamorphism and diagenesis; and (3) a late-stage, very low temperature (<100°C) alteration of the ultramafic bodies by meteoric ground waters, producing lizardite-chrysotile serpentine. In the first two cases, the pore fluid involved in the alteration of the Franciscan rocks was sea water. However, this water became somewhat depleted in D and enriched in 18O during blueschist metamorphism, evolving to values of δD ≈ ? 20 and δ18O ≈ + 6 to + 8 at the highest grades. Except for one graywacke sample, the meteoric waters that affected the serpentinites did not significantly change the DH ratios of the OH-bearing minerals in any other Franciscan rock.The δ18O values of orogenic andesites are too low for such magmas to have formed by direct partial melting of Franciscan-type materials in a subduction zone. Andesites either form in some other fashion, or the melts must undergo thorough isotopic exchange with the upper mantle. The great Cordilleran granodiorite-tonalite batholiths, however, are much richer in 18O and may well have formed by large-scale melting or assimilation of Franciscan-type rocks. The range of δD values of Franciscantype rocks is identical to the ?50 to ?80 range shown by most igneous rocks. This suggests that ‘primary magmatic H2O’ throughout the world may be derived mainly by partial melting of Franciscantype materials, or by dehydration of such rocks in the deeper parts of a Benioff zone.  相似文献   

13.
《Gondwana Research》2001,4(3):377-386
The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite facies terrain in southern India comprising garnet-biotite gneiss, garnet-sillimanite gneiss and orthopyroxene granulites as major rock types. Calc-silicate rocks and marbles, occurring as minor lithologies in the Kerala Khondalite Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T-fluid history dominated by internal fluid buffering during the peak metamorphism, followed by external fluid influx during decompression. The carbon and oxygen isotopic compositions of calcite from three representative metacarbonate localities show contrasting evolutionary trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratios (δ13C ∼ 0‰ and δ18O ∼ 20‰) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The δ13C and δ18O values of ∼ −9‰ and 11‰, respectively, for calcite from calc-silicate rocks at Nuliyam are considerably low and heterogeneous. The wollastonite formation here, possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism, which resulted in decarbonation and isotope resetting. Further, petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the calc-silicate rocks, with subsequent isotope resetting, more towards the contact between calc-silicate rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (δ18O ∼ 13‰) and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx, supported by petrologic evidence. The stable isotope signatures obtained from the individual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.  相似文献   

14.
This paper investigates the origin of low-δ18O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ18O values (average 8.0‰, ± 0.7, n?=?33) to the granite (average 8.3?±?1.0, n?=?7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ18O values (average 1.6‰, ± 2.1, n?=?55). In some cases quartz phenocrysts have δ18O values as low as ? 2.5‰. The variation in δ18O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ18O values have been affected by fluid–rock interaction. Based on a ?quartz?magma value of 0.6‰, magma δ18O values must have been as low as ? 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ18O values that range from +?1.1 to +?4.6‰, and ? 2.3 to +?5.6‰, respectively. These δ18O values correlate negatively (r = ? 0.96) with initial 87Sr/86Sr, which can be explained by the event that lowered δ18O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by 18O-depleted meteoric water during global glaciation at ~?550 Ma. The early melts had variable δ18O value but as melt pockets interconnected during melting, the δ18O values approached that of average gneiss. Variable quartz phenocryst δ18O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ18O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.  相似文献   

15.
The Jiapigou gold belt (>150 t Au), one of the most important gold-producing districts in China, is located at the northeastern margin of the North China Craton. It is composed of 17 gold deposits with an average grade around 10 g/t Au. The deposits are hosted in Archean gneiss and TTG rocks, and are all in shear zones or fractures of varying orientations and magnitudes. The δ34S values of sulfide from ores are mainly between 2.7?‰ and 10?‰. The Pb isotope characteristics of ore sulfides are different from those of the Archean metamorphic rocks and Mesozoic granites and dikes, and indicate that they have different lead sources. The sulfur and lead isotope compositions imply that the ore-forming materials might originate from multiple, mainly deep sources. Fluid inclusions in pyrite have 3He/4He ratios of 0.6 to 2.5 Ra, whereas their 40Ar/36Ar ratios range from 1,444 to 9,805, indicating a dominantly mantle fluid with a negligible crustal component. δ18O values calculated from hydrothermal quartz are between ?0.2?‰ and +5.9?‰, and δD values of the fluids in the fluid inclusions in quartz are from ?70?‰ to ?96?‰. These ranges suggest dominantly magmatic water with a minor meteoric component. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore-forming fluids have a dominantly mantle source with minor crustal addition.  相似文献   

16.
ABSTRACT

The adakitic dikes with zircon ages of ~120–118 Ma are identified in the Xiejiagou gold deposit, Jiaobei terrain. The Early Cretaceous intermediate-acidic dikes include monzonite, quartz monzonite, and monzogranite, which have SiO2, Al2O3, K2O, Na2O contents ranging from 61.01 to 74.72 wt. %, 14.42 to 17.28 wt.%, 2.55 to 4.45 wt.%, and 3.09 to 4.64 wt.%, respectively. The dikes are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr, and Ba) and light rare earth elements (LREEs), but are depleted in high field strength elements (HFSEs) (e.g., Nb, Ta) and heavy rare earth elements (HREEs). They are also characterized by continental crust-like isotopic compositions: whole-rock δ18O = 7.6–9.9‰, (87Sr/86Sr)i = 0.70893–0.71036, and εNd(t) = ?13.16 to ?17.06. Assimilation and fractional crystallization are limited, and partial melting of source rock is the main mechanism in controlling the compositional variation. The high Sr/Y (41.51–93.25) and (La/Yb)N (22.9–44.7) ratios imply that the dikes have affinities similar to adakitic rocks. The regional geological setting, coupled with the geochemical and isotopic data, indicate that the adakitic dikes were unlikely produced by partial melting of a subducted oceanic slab or a thickened/delaminated lower crust, or AFC processes of mantle-derived magmas. We conclude that the adakitic dikes are generated by partial melting of lower crust which was triggered and modified by underplating of the lithospheric mantle. We also propose that the eastern part of the North China Craton experienced severe thinning in the Early Cretaceous (~135–120 Ma), and the lithospheric thinning is related to delamination of the lower crust.  相似文献   

17.
Nineteen samples of metamorphosed carbonate-bearing rocks were analyzed for carbon and oxygen isotope ratios by ion microprobe with a ∼5-15 μm spot, three from a regional terrain and 16 from five different contact aureoles. Contact metamorphic rocks further represent four groups: calc-silicate marble and hornfels (6), brucite marble (2), samples that contain a reaction front (4), and samples with a pervasive distribution of reactants and products of a decarbonation reaction (4). The average spot-to-spot reproducibility of standard calcite analyses is ±0.37‰ (2 standard deviations, SD) for δ18O and ±0.71‰ for δ13C. Ten or more measurements of a mineral in a sample that has uniform isotope composition within error of measurement can routinely return a weighted mean with a 95% confidence interval of 0.09-0.16‰ for δ18O and 0.10-0.29‰ for δ13C. Using a difference of >6SD as the criterion, only four of 19 analyzed samples exhibit significant intracrystalline and/or intercrystalline inhomogeneity in δ13C at the 100-500 μm scale, with differences within individual grains up to 3.7‰. Measurements are consistent with carbon isotope exchange equilibrium between calcite and dolomite in five of six analyzed samples at the same scale. Because of relatively slow carbon isotope diffusion in calcite and dolomite, differences in δ13C can survive intracrystalline homogenization by diffusion during cooling after peak metamorphism and likely represent the effects of prograde decarbonation and infiltration. All but 2 of 11 analyzed samples exhibit intracrystalline differences in δ18O (up to 9.4‰), intercrystalline inhomogeneity in δ18O (up to 12.5‰), and/or disequilibrium oxygen isotope fractionations among calcite-dolomite, calcite-quartz, and calcite-forsterite pairs at the 100-500 μm scale. Inhomogeneities in δ18O and δ13C are poorly correlated with only a single mineral (dolomite) in a single sample exhibiting both. Because of relatively rapid oxygen isotope diffusion in calcite, intracrystalline inhomogeneities in δ18O likely represent partial equilibration between calcite and fluid during retrograde metamorphism. Calcite is in oxygen isotope exchange equilibrium with forsterite in one of four analyzed samples, in equilibrium with dolomite in none of six analyzed samples, and in equilibrium with quartz in neither of two analyzed samples. There are no samples of contact metamorphic rock with analyzed reactants and products of an arrested metamorphic reaction that are in oxygen isotope equilibrium with each other. The degree of departure from equilibrium in analyzed samples is variable and is often related, at least in part, to alteration of δ18O of calcite during retrograde fluid-rock reaction. In situ sub-grain-scale carbon and oxygen isotope analyses of minerals are advisable in the common applications of stable isotope geochemistry to metamorphic petrology. Correlation of sub-mm scale stable isotope data with imaging will lead to improved understanding of reaction kinetics, reactive fluid flow, and thermal histories during metamorphism.  相似文献   

18.
Lithium isotope signatures of whole rock pegmatite samples and mineral separates from the rare element-bearing Little Nahanni Pegmatite Group, NWT, and whole rock samples from nearby granitic intrusions were measured. Correlation of the Li isotopic values from the pegmatite dikes with whole rock trace element geochemistry, mineralogy and primary textural evidence reflect mechanisms of Li isotopic fractionation during pegmatite formation. The heavier δ7Li signatures within the broad range measured from whole rock LNPG samples (? 0.94‰ to + 11.36‰) are related to the consolidation of the final ~ 15% melt fraction of a volatile-rich peraluminous magma in the late stages of magmatic fractionation. Rock-forming minerals (quartz, albite, spodumene and mica) display δ7Li signatures that indicate consolidation of the dikes under variable, non-equilibrium conditions. Lithium isotope signatures of relatively cool, highly evolved peraluminous magmas reflect the build-up of fluxes (e.g., H2O and F) and provide a qualitative assessment of the state of mineral/melt chemical equilibrium.  相似文献   

19.
Sulfide mineralization in the Voisey’s Bay Intrusion, Labrador, Canada, is closely associated with country rock xenoliths that have extensively reacted with basaltic magma. In order to better understand the processes that control the assimilation of country rocks by mafic magma, a detailed study of oxygen isotope systematics related to magma-country rock interaction in the Voisey’s Bay area was undertaken. Protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths (2-10 cm in diameter) composed of Ca-rich plagioclase, corundum, hercynite, and minor magnetite. Overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix. The δ18O values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are: plagioclase (8.7-12.3‰), orthoclase (9.5-9.8‰), biotite (5.2-8.7‰), garnet (8.3-10.8‰), pyroxene (8.0-10.1‰), and quartz (9.6-14.0). The δ18O values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths. Hercynite, plagioclase, and corundum separates from the xenoliths have δ18O values that vary from 2.9‰ to 10.5‰, 5.6‰ to 10.9‰, and 2.0‰ to 6.8‰, respectively. Although a siliceous 18O-enriched melt has been lost from the xenoliths, corundum, and feldspar δ18O values are significantly lower than expected through melt loss alone. The relatively low δ18O values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a δ18O value of ∼5.5‰ to 6.0‰. The high-18O melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths (δ18O values from 6.2‰ to 10.7‰), and in hercynite that replaced corundum. However, mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system. δ18O and δD values of biotite surrounding the plagioclase overgrowth range from 5.0‰ to 6.2‰ and −58‰ to −80‰, respectively. These data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma. The current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high-18O partial melt from the xenoliths. The feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma. The minerals within the xenoliths are not in oxygen isotopic equilibrium with each other, due in part to rapid thermal equilibration, partial melting, and partial exchange with flow through magma.  相似文献   

20.
Geologic, petrographic and petrochemical studies of the late Mesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra-high potassic, shoshonitic and high potassic calc-alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr)i=0.70895-0.71140) and low (143Nd/144Nd)i ratios (varying from 0.51135 to 0.51231); and its δ18OSMow. whole rock values vary from +5.8‰ to +10.6‰ with a mean of+7.1‰. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced metasomatism by mantle-derived fluids with H2O-dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low-degree partial melting of the enri  相似文献   

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