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1.
Composite dikes, consisting of aphyric basaltic margins and phenocryst-rich rhyolitic interiors, cut the Gouldsboro granite of coastal Maine at many localities. Limited hybridization (exchange of crystals, commingling, and mixing) occurs in most of the dikes and indicates that the two magmas were contemporaneous with emplacement of rhyolitic magma following closely in time the initial emplacement of the basaltic dike. Petrographic characteristics and geochemistry indicate that the source of the rhyolite was resident magma in the Gouldsboro granite magma chamber. The composite dikes formed when basaltic dikes ruptured the Gouldsboro magma chamber, permitting partly crystallized magma from the margin of the chamber to flow outward into the center of the basaltic dikes. Field relations of similar composite dikes in other areas (e.g., Iceland, Scotland) are consistent with this model. A second type of composite dike (silicic margins with chilled basaltic pillows) commonly cuts mafic intrusions along the Maine coast and probably formed when a granitic dike ruptured an established chamber of mafic magma, permitting resident mafic magma to collapse downward into the still Liquid granitic dike. Most composite dikes have probably formed when a magma chamber was disrupted by a dike of contrasting magma rather than by tapping a stratified magma chamber. 相似文献
2.
Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; εNd (at 370 Ma)=−2.7 to −0.7; initial 207Pb/ 204Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; εNd (at 370 Ma)=−7.0 to −0.6; initial 207Pb/ 204Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; εNd (at 370 Ma)=−2.6 to +0.3; initial 207Pb/ 204Pb 15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are 30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures. Derivation of the monzodioritic rocks of the Mooselookmeguntic igneous complex most likely occurred by melting of Bronson Hill belt crust of mafic to intermediate composition. The Mooselookmeguntic igneous complex granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of initial Nd and Pb isotopic compositions with data for Central Maine belt metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this crustal package or through a mixture of metasedimentary rocks with magmas derived from Bronson Hill belt crust. New data from other granites in western Maine include Pb isotope data for the Phillips pluton, which permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the Central Maine belt, but suggest that granodiorites were extracted from sources more similar to Bronson Hill belt crust. Data for the Redington pluton are best satisfied by generation from sources in either the Bronson Hill belt or Laurentian basement. Based on these data, we infer that Bronson Hill belt crust was more extensive beneath the Central Maine belt than previously recognized and that mafic melts from the mantle were not important to genesis of Devonian granite magma. 相似文献
3.
The major and trace elements and Sr–Nd–Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at 43 Ma at shallow depths ( 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO 2 = 56.98–72.67 wt.%; Mg# = 36.8–50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO 2 = 53.21–60.94 wt.%; Mg# = 44.4–53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb) N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd–Sr isotopic compositions for the host rocks are εNd(43 Ma) = − 0.6 to 0.8 and mostly ISr = 0.70482–0.70548. The Nd model ages ( TDM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are ( 206Pb/ 204Pb) = 18.60–18.65, ( 207Pb/ 204Pb) = 15.61–15.66 and ( 208Pb/ 204Pb) = 38.69–38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with ISr ranging from 0.70485 to 0.70517, εNd(43 Ma) − 0.1 to 0.8 and with Pb isotopic ratios of ( 206Pb/ 204Pb) = 18.58–18.64, ( 207Pb/ 204Pb) = 15.60–15.66 and ( 208Pb/ 204Pb) = 38.64–38.77. The MMEs have TDM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr–Nd isotope data indicates that 77–83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime. 相似文献
4.
This review, in honor of Ilmari Haapala's retirement, reflects on lessons learned from studies of three granitic systems in western North America: (1) Mesoproterozoic samples from west Texas and east New Mexico; (2) Laramide granitic systems associated with porphyry-copper deposits in Arizona; and (3) granites of the Colorado Mineral Belt. The studies elucidate relationships amongst tectonic setting, source material, and magma chemistry. Mesoproterozoic basement samples are from two different felsic suites with distinct elemental and isotopic compositions. The first suite, the “plutonic province”, is dominantly magnesian, calc-alkalic to alkali-calcic, and metaluminous. It has low K2O/Na2O and Rb/Sr, and Nd model ages of 1.56 to 1.40 Ga. The second suite, the “Panhandle igneous complex”, is magnesian, metaluminous, alkalic, and is part of the Mesoproterozoic belt of magmatism that extends from Finland to southwestern United States. Samples from the Panhandle igneous complex demonstrate three episodes of magmatism: the first pulse was intrusion of quartz monzonite at 1380 to 1370 Ma; the second was comagmatic epizonal granite and rhyolite at 1360 to 1350 Ma. Both of these rock types are high-K to slightly ultra-high-K. The third pulse at 1338 to 1330 Ma was intrusion of ultra-high-K quartz syenite. Nd model ages (1.94 to 1.52 Ga) are distinct from those of the “plutonic province” and systematically older than crystallization ages, implying a substantial crustal input to the magmas. At the Sierrita porphyry-copper deposit in the Mazatzal Province of southeastern Arizona, trace element, Sr, and Nd isotopic compositions were determined for a suite of andesitic and rhyolitic rocks (67 Ma) intruded by granodiorite and granite. Isotopic composition and chemical evolution are well correlated throughout the suite. Andesite has the least negative initial εNd (−4.3) and lowest 87Sr/86Sri (0.7069). It is also the oldest and chemically most primitive, having low concentrations of Rb, SiO2, and high concentrations of transition elements. These parameters change through the system to the youngest unit (granite), which has the most negative εNd (−8.5), the highest 87Sr/86Sri (0.7092), and is chemically most evolved. Correlation between chemical and Nd isotopic evolution probably resulted from a continuous process of progressive assimilation, in which mafic magmas invade and incorporate continental crust. Deposits in Arizona with εNd values more negative than the −8.5 of Sierrita lie in the older Yavapai province in the northwestern part of the state. The difference in the most negative epsilon Nd implies that Nd isotopic signature is sensitive to the age of the Precambrian domain. The granites from the Colorado Mineral Belt were emplaced during the transition from Laramide convergence to mid-Tertiary extension. Three different groups of granites are recognized. The first is Laramide and was formed during assimilation-fractional crystallization involving lower crustal mafic source materials; the second and third groups are mid-Tertiary and represent intracrustal melting of heterogeneous sources. This change in source regions and melt regimes in transition from convergence to extension is fundamental to the Mesozoic and Cenozoic evolution of western North America. 相似文献
5.
Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648. The initial 143Nd/144Nd ratios, expressed by εNd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally. 相似文献
6.
The role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ? Hf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ? Hf in two of the three analysed samples (8.8 and 10.1 ? Hf units) exceeds that expected from a single homogeneous population (～4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ? Nd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/ 86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites. 相似文献
7.
Magma mingling has been identified within the continental margin of southeastern China.This study focuses on the relationship between mafic and felsic igneous rocks in composite dikes and plutons in this area,and uses this relationship to examine the tectonic and geodynamic implications of the mingling of mafic and felsic magmas.Mafic magmatic enclaves(MMEs) show complex relationships with the hosting Xiaocuo granite in Fujian area,including lenticular to rounded porphyritic microgranular enclaves containing abundant felsic/mafic phenocrysts,elongate mafic enclaves,and back-veining of the felsic host granite into mafic enclaves.LA-ICP-MS zircon U-Pb analyses show crystallization of the granite and dioritic mafic magmatic enclave during ca.132 and 116 Ma.The host granite and MMEs both show zircon growth during repeated thermal events at-210 Ma and 160-180 Ma.Samples from the magma mingling zone generally contain felsic-derived zircons with well-developed growth zoning and aspect ratios of 2-3,and maficderived zircons with no obvious oscillatory zoning and with higher aspect ratios of 5-10.However,these two groups of zircons show no obvious trace element or age differences.The Hf-isotope compositions show that the host granite and MMEs have similar ε_(Hf)(t) values from negative to positive which suggest a mixed source from partial melting of the Meso-Neoproterozoic with involvement of enriched mantlederived magmas or juvenile components.The lithologies,mineral associations,and geochemical characteristics of the mafic and felsic rocks in this study area indicate that both were intruded together,suggesting Early Cretaceous mantle—crustal interactions along the southeastern China continental margin.The Early Cretaceous magma mingling is correlated to subduction of Paleo-Pacific plate. 相似文献
8.
Mafic enclaves are commonly found in intermediate arc magmas, and their occurrence has been linked to eruption triggering by pre-eruptive magma mixing processes. New major, trace, Sr–Nd and U–Th isotope data of rocks from Nisyros in the Aegean volcanic arc are presented here. Pre-caldera samples display major and trace element trends that are consistent with fractionation of magnetite and apatite within intermediate compositions, and zircon within felsic compositions, and preclude extensive hybridization between mafic and felsic magmas. In contrast, post-caldera dacites form a mixing trend towards their mafic enclaves. In terms of U-series isotopes, most samples show small 238U excesses of up to 10%. Mafic enclaves have significantly higher U/Th ratios than their dacitic host lavas, precluding simple models that relate the mafic and felsic magmas by fractionation or aging alone. A more complicated petrogenetic scenario is required. The post-caldera dacites are interpreted to represent material remobilized from a young igneous protolith following influx of fresh mafic magma, consistent with the U–Th data and with Sr–Nd isotope constraints that point to very limited (< 10%) assimilation of old crust at Nisyros. When these results are compared to data from Santorini in the same arc, there are many geochemical similarities between the two volcanic centers during the petrogenesis of the pre-caldera samples. However, striking differences are apparent for the post-caldera lavas: in Nisyros, dacites show geochemical and textural evidence for magma mixing and remobilization by influx of mafic melts, and they erupt as viscous lava domes; in Santorini, evidence for geochemical hybridization of dacites and mafic enclaves is weak, dacite petrogenesis does not involve protolith remobilization, and lavas erupt as less viscous flows. Despite these differences, it appears that mafic enclaves in intermediate Aegean arc magmas consistently yield timescales of at least 100 kyrs between U enrichment of the mantle wedge and eruption, on the upper end of those estimated for the eruptive products of mafic arc volcanoes. Finally, the data presented here provide constraints on the rates of differentiation from primitive arc basalts to dacites (less than 140 kyrs), and on the crustal residence time of evolved igneous protoliths prior to their remobilization by mafic arc magmas (greater than 350 kyrs). 相似文献
9.
Silurian plutonic suites in the Newfoundland Appalachians include abundant gabbro, monzogabbro and granite to granodiorite and lesser quartz diorite and tonalite. Most are medium- to high-K, but included are some low-K and shoshonitic mafic compositions. Felsic rocks are of both alkaline (A-type or within-plate granite (WPG)) and calc-alkaline volcanic arc granite (VAG) affinity. Mafic rocks include both arc-like (Nb/Th < 3) calc-alkaline and non-arc-like (Nb/Th > 3) transitional calc-alkaline basalt to continental tholeiitic affinity compositions. εNd( T) values range from − 9.6 to + 5.4 and δ18O (VSMOW) values range from + 3.1 to + 13.2‰. A rapid progression from exclusively arc-type to non-arc-like mafic and then contemporaneous WPG plus VAG magmatism has been documented using precise U–Pb zircon dating. Earlier arc-like plutonism indicates subduction, while asthenosphere-derived mafic magmas support slab break-off, due to subduction of a young, warm back-arc basin. Contemporaneous mafic magmas with arc and non-arc geochemical signatures may reflect tapping of asthenospheric and subcontinental lithospheric mantle (SCLM) sources and/or contamination of asthenosphere-derived magmas by SCLM or crust. The brevity (< 5 Ma) of the mafic magmatic pulse agrees with the transient nature of magmatism associated with slab break-off. The subsequent ca. 1 to 2 m.y. period of voluminous WPG and VAG plutonism likely reflects mafic magma-driven partial melting of both SCLM and crustal sources, respectively. Continuation of VAG-like magmatism for an additional 2 to 5 m.y. may reflect lower solidus temperatures of crustal materials, enabling anatexis to continue after mantle melting ceased. East to west spatial variation of εNd and (La/Yb)CN in Silurian plutons suggests a transition from shallow melting of juvenile sources proximal to the collision zone to deeper melting of old source materials in the garnet-stability field further inboard. Previous work has demonstrated that geochemical discriminaton of post-collisional granitoid magmatism (PCGM) is difficult in the absence of other constraints. Our example should contribute to the understanding and identification of PCGM if it can be employed as a ‘fingerprint’ for slab break-off-related PCGM within the Paleozoic geological record. 相似文献
10.
New Sr and Nd isotopic data are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bridle Track plutons, along with analyses of their host enclave groundmass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Feldspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave groundmass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equilibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher 143Nd/ 144Nd (i) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone. 相似文献
11.
The Pleasant Bay layered gabbro-diorite complex (420 Ma) formed via repeated injections of mafic magma into a felsic magma chamber. It is dominated by repeating sequences (macrorhythmic units) with chilled gabbroic bases which may grade upward into medium-grained gabbro, diorite and granite. Each unit represents an injection of mafic magma into the chamber followed by differentiation. Increases in Sr i and decreases in ) Ndi with stratigraphic height indicate open-system isotopic behaviour and exchange between the mafic and felsic magmas. Isotopic variations of whole-rock samples in individual macrorhythmic units do not conform to bulk mixing or AFC models between potential parental magmas. Sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important, that some of the rocks contain feldspar xenocrysts, and that the rocks are isotopically heterogeneous on an intercrystal scale. Xenocryst abundance increases with stratigraphic height, suggesting that crystal exchange occurred in situ. The lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer. Mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes (representing the parental melts) and suggest rapid in-situ diffusional exchange following emplacement of individual mafic replenishments. 相似文献
12.
The Zhongchuan district is an important component of the metallogenic belt in the Western Qinling. The Zhongchuan granite pluton occurring in the centre of the Zhongchuan metallogenic area has been poorly constrained, though the Triassic granite in Western Qinling has been well documented. In‐situ zircon U–Pb ages, Hf isotopic compositions and whole‐rock geochemical data are presented for host granite and mafic microgranular enclaves (MMES) from the Zhongchuan pluton, in order to constrain its sources, petrogenesis and tectonic setting of the pluton. The distribution of major, trace and rare earth elements apparently reflect exchange between the MMES and the host granitic rocks mainly due to interactions between coeval felsic host magma and mafic magma. The zircon U–Pb age of host granite (231.6 ± 1.5 to 235.8 ± 2.3 Ma) has overlapping uncertainty with that of the MMES (236.6 ± 1.3 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the MMES ( εHf(t) = −13.4 to 4.0) is distinct from the host granite ( εHf(t) = −15.7 to 0.0), indicating that both enriched subcontinental lithosphere mantle (SCLM) and crustal sources contributed to their origin. The zircons have two‐stage Hf model ages of 1064 to 1798 Ma for the host granite and 858 to 1747 Ma for the MMES. This suggests that the granitic pluton was likely derived from partial melting of a Late Mesoproterozoic crust, with subsequent interaction with the SCLM‐derived mafic magmas in tectonic affinity to the South China Block. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
13.
The Tunk Lake pluton of coastal Maine, USA is a concentrically zoned granitic body that grades from an outer hypersolvus granite
into subsolvus rapakivi granite, and then into subsolvus non-rapakivi granite, with gradational contacts between these zones.
The pluton is partially surrounded by a zone of basaltic and gabbroic enclaves, interpreted as quenched magmatic droplets
and mushes, respectively, as well as gabbroic xenoliths, all hosted by high-silica granite. The granite is zoned in terms
of mineral assemblage, mineral composition, zircon crystallization temperature, and major and trace element concentration,
from the present-day rim (interpreted as being closer to the base of the chamber) to the core (interpreted as being closer
to the upper portions of the chamber). The ferromagnesian mineral assemblage systematically changes from augite and hornblende
with augite cores in the outermost hypersolvus granite to hornblende, to hornblende and biotite, and finally, to biotite only
in the subsolvus granite core of the pluton. Sparse fine-grained basaltic enclaves that are most common in the outermost zone
of the pluton suggest that basaltic magma was present in the lower portions of the magma chamber at the same time that the
upper portions of the magma chamber were occupied by a granitic crystal mush. However, the slight variations in initial Nd
isotopic ratio in granites from different zones of the pluton suggest that contamination of the granitic melt by basaltic
melt played little role in generating the compositional gradation of the pluton. The zone of basaltic and gabbroic chilled
magmatic enclaves, and gabbroic xenoliths, hosted by high-silica granite, that partially surround the pluton is interpreted
as mafic layers at the base of the pluton that were disrupted by invading late-stage high-silica magma. These mafic layers
are likely to have consisted of basaltic lava layers and basalt that chilled against granitic magma to produce coarse-grained
gabbroic mush. Basaltic and gabbroic magmatic enclaves and gabbroic xenoliths are hornblende-bearing, suggesting that their
parent melts were relatively hydrous. The water-rich nature of the underplating mafic magmas may have prevented extensive
invasion of the granitic magma by these magmas, owing to the much greater viscosity of the granitic magma than the mafic magmas
in the temperature range over which magma interaction could have occurred. 相似文献
14.
At Telões, a subaluminous medium- to coarse-grained porphyritic biotite granite, crops out along the Vila Real NNE–SSW fault. It is a post-tectonic granite of 299±3 Ma old given by U–Pb isotopic data on zircon. It contains metaluminous to subaluminous tonalitic, granodioritic and monzogranitic enclaves. All granitoids have Fe 2+-biotite and some enclaves contain magnesiohornblende and subsolidus actinolite. Monzogranitic enclaves show obvious similarities to the host granite. Linear array between enclaves and host granite is observed in Rb–Sr typical isochron diagram and gives the age of 286±11 Ma and ( 87Sr/ 86Sr) 0=0.7063±0.0011. Microgranular enclaves have δ18O values similar to those of the hosting granite. Microgranular enclaves are hybrid rocks probably formed by mixing between a tonalitic enclave magma and a host granite magma as supported by the modelling of major and trace elements. The similar isotopic signatures suggest a subsequent partial equilibration of the enclaves and granite magmas. 相似文献
15.
The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61?wt.% SiO 2), while the upper part is occupied by fine-grained granite (69.80–70.57?wt.% SiO 2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes. 相似文献
16.
The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/ 86Sr ratios (0.7065–0.7085) and lower ε Nd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there. 相似文献
17.
The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern
unit is relatively mafic (SiO 2∼64%) and lacks cordierite. The more felsic central and southern units (SiO 2=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular
enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains.
Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO 2=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma.
Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only
primary mafic phase (SiO 2=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m
thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams,
samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin
by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high
Mg#, low Na 2O, ɛ Nd=−1.2 to −5.1, 87Sr/ 86Sr=0.7055 to 0.7080, δ 18O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous
enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by
the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛ Nd=−4.5 to −6.8, 87Sr/ 86Sr=0.7071 to 0.7115, δ 18O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic
sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of
Al 2O 3/(MgO+FeO tot) and K 2O/Na 2O, in combination with low ratios of CaO/(MgO+FeO tot), variable values of Sr/Nd, Eu/Eu*[=Eu cn/(Sm cn × Gd cn) 0.5] and (Tb/Yb) cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic
enclaves (ɛ Nd=−4.6 to −5.2; 87Sr/ 86Sr=0.7060 to 0.7073; δ 18O ∼8.1‰) are similar to those of the low ɛ Nd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial
87Sr/ 86Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high 87Sr/ 86Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated
by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas
was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation
of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones.
Received: 30 April 1999 / Accepted: 6 August 1999 相似文献
18.
阿尔金造山带南缘玉苏普阿勒克塔格岩体中的似斑状中粗粒黑云钾长花岗岩发育有岩浆成因的暗色包体,并且该花岗岩被花岗细晶岩呈脉状侵入。该岩体含有丰富的岩浆混合作用特征: 如暗色包体中的碱性长石斑晶、针状磷灰石、长石的环斑结构、石英/斜长石主晶和榍石眼斑等。暗色包体、寄主花岗岩和花岗细晶岩代表了岩浆混合演化过程中不同端元比例混合的产物。地球化学特征上,钾长花岗岩和暗色包体的主要氧化物含量在Harker图解中多呈线性变化。暗色包体主要为闪长质,MgO、K 2O含量高,为钾玄岩系列,总体上高场强元素不亏损,显示了岩浆混合中的基性端元信息,可能为幔源熔体结晶分异或壳幔物质的混合产物。寄主花岗岩均为准铝质,富碱,为高钾钙碱性系列,亏损Nb、Ta、Sr、P、Ti等高场强元素,高K 2O/Na 2O,富集高不相容元素,Ga含量高,显示了A型花岗岩的特征,Th/U 和Nb/Ta比值分别介于为6.67~10.96、8.99~11.94,代表了下地壳源区。花岗细晶岩均为钠质、过铝质,TiO 2、MgO含量低, Na 2O和CaO含量高,具有混合岩浆侵位后分异的特征。岩相学和地球化学特征说明岩浆混合作用对于环斑结构花岗岩的形成起到重要作用。花岗细晶岩中环斑长石的斜长石外环与钾长石内核的厚度比大于钾长花岗岩中的环斑长石,指示混合岩浆在一定的减压条件下更有利于环斑结构的形成。玉苏普阿勒克塔格岩体中的钾玄质暗色包体、高钾钙碱性花岗岩和中钾钙碱性花岗细晶岩代表了岩浆演化不同阶段的产物,反映了一个幔源岩浆和下地壳不断相互作用,引起地壳连续伸展减薄的过程,指示阿尔金南缘在早古生代末期存在造山后伸展背景下的幔源岩浆底侵作用。同一岩体中两种不同时代岩性的环斑结构显示了该岩体形成历史中的一定时空演化关系,代表了伸展过程中不同阶段的产物。 相似文献
19.
Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from 45–75 wt.% SiO 2. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate. Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting. 相似文献
20.
华北地台北缘乌拉山地区的沙德盖钾长花岗岩体中普遍发育以二长岩为主的暗色微粒包体,包体具塑性流变特征,与寄主岩的接触界线或为截然或为渐变过渡。岩相学观察表明,包体中发育多种反映岩浆混合作用的典型组构,如石英眼斑、环斑长石、镁铁质团块、钾长石巨晶的溶蚀、磷灰石的针柱状形貌、长石中的包体带以及钙长石的"针尖"结构等。造岩矿物的电子探针分析表明,岩浆混合在沙德盖岩体的形成中起了重要作用,寄主花岗岩浆主要来自下地壳,而暗色包体岩浆则主要为地幔来源。锆石LA-ICP-MS U-Pb同位素定年结果显示,沙德盖花岗岩及其暗色微粒包体的形成时代基本一致,分别为233.4±2.3Ma和229.7±1.5Ma(中三叠世),进一步佐证了该岩体是岩浆混合作用的产物。研究认为,当铁镁质岩浆与长英质岩浆混合时,早期基性岩浆的快速淬冷形成了边界清楚、具明显冷凝边且暗色矿物含量较高的包体;随着两种不同成分岩浆之间温差的减小以及组分的交换,进一步形成了颜色较浅、边界渐变过渡和无明显冷凝边的包体。 相似文献
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