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1.
—We investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern Japan. The comparison of the seismic tomography with laboratory velocity data on a partially-molten mantle rock yields estimates of melting zones in three dimensions. We employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature. Melting and magma-rich zones derived from the velocity structure coincide with observed low Q zones. The results of the three-dimensional mapping indicate that the source of magma in island arc is diapir-like melting patches localized within the low velocity zones of the mantle wedge. Extensive volcanic activity along the volcanic front is due to the presence of vast magma-rich zones just beneath the Moho. Those melting zones in the uppermost mantle may, in turn, cause melting of lower crustal materials and produce felsic magma. Melt appears to stay at and beneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-arc region, which may correlate with across-arc variations of chemical compositions of the volcanic rocks observed in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors. 相似文献
2.
Silicate‐oxide mineral chemistry of mafic–ultramafic rocks as an indicator of the roots of an island arc: The Chilas Complex,Kohistan (Pakistan) 
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下载免费PDF全文 The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide. 相似文献
3.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma. 相似文献
4.
The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites. 相似文献
5.
The origin of Arenal basaltic andesite can be explained in terms of fractional crystallization of a parental high-alumina basalt (HAB), which assimilates crustal rocks during its storage, ascent and evolution. Contamination of this melt by Tertiary calc-alkalic intrusives (quartz–diorite and granite, with 87Sr/86Sr ratios ranging 0.70381–0.70397, nearly identical with those of the Arenal lavas) occurs at upper crustal levels, following the interaction of ascending basaltic magma masses with gabbroic–anorthositic layers. Fragments of these layers are found as inclusions within Arenal lavas and tephra and may show reaction rims (1–5 mm thick, consisting of augite, hypersthene, bytownitic–anorthitic plagioclase, and granular titanomagnetite) at the gabbro–lava interface. These reaction rims indicate that complete `assimilation' was prevented since the temperature of the host basaltic magma was not high enough to melt the gabbroic materials (whose mineral phases are nearly identical to the early formed liquidus phases in the differentiating HAB). Olivine gabbros crystallized at pressure of about 5–6 kbar and equilibrated with the parental HAB at pressures of 3–6 kbar (both under anhydrous and hydrous conditions), and temperatures ranging 1000–1100°C. In particular, `deeper' interactions between the mafic inclusions and the hydrous basaltic melt (i.e., with about 3.5 wt.% H2O) are likely to occur at 5.4 (±0.4) kbar and temperatures approaching 1100°C. The olivine gabbros are thus interpreted as cumulates which represent crystallized portions of earlier Arenal-type basalts. Some of the gabbros have been `mildly' tectonized and recrystallized to give mafic granulites that may exhibit a distinct foliation. Below Arenal volcano a zoned magma chamber evolved prior the last eruptive cycle: three distinct andesitic magma layers were produced by simple AFC of a high-alumina basalt (HAB) with assimilation of Tertiary quartz–dioritic and granitic rocks. Early erupted 1968 tephra and 1969 lavas (which represent the first two layers of the upper part of a zoned magma chamber) were produced by simple AFC, with fractionation of plagioclase, pyroxene and magnetite and concomitant assimilation of quartz–dioritic rocks. Assimilation rates were constant (r1=0.33) for a relative mass of magma remaining of 0.77–0.80, respectively. Lavas erupted around 1974 are less differentiated and represent the `primitive andesitic magma type' residing within the middle–lower part of the chamber. These lavas were also produced by simple AFC: assimilation rates and the relative mass of magma remaining increased of about 10%, respectively (r1=0.36, and F=0.89). Ba enrichment of the above lavas is related to selective assimilation of Ba from Tertiary granitic rocks. Lava eruption occurred as a dynamic response to the intrusion of a new magma into the old reservoir. This process caused the instability of the zoned magma column inducing syneruptive mixing between portions of two contiguous magma layers (both within the column itself and at lower levels where the new basalt was intruded into the reservoir). Syneruptive mixing (mingling) within the middle–upper part of the chamber involved fractions of earlier gabbroic cumulitic materials (lavas erupted around 1970). On the contrary, within the lower part of the chamber, mixing between the intruded HAB and the residing andesitic melt was followed by simple fractional crystallization (FC) of the hybrid magma layer (lavas erupted in 1978–1980). By that time the original magma chamber was completely evacuated. Lavas erupted in 1982/1984 were thus modelled by means of `open system' AFCRE (i.e., AFC with continuous recharge of a fractionating magma batch during eruption): in this case assimilation rates were r1=0.33 and F=0.86. Recharge rates are slightly higher than extrusion rates and may reflect differences in density (between extruded and injected magmas), together with dynamic fluctuations of these parameters during eruption. Ba and LREE (La, Ce) enrichments of these lavas can be related to selective assimilation of Tertiary granitic and quartz–dioritic rocks. Calculated contents for Zr, Y and other REE are in acceptable agreement with the observed values. It is concluded that simple AFC occurs between two distinct eruption cycles and is typical of a period of repose or mild and decreasing volcanic activity. On the contrary, magma mixing, eventually followed by fractional crystallization (FC) of the hybrid magma layer, occurs during an ongoing eruption. Open-system AFCRE is only operative when the original magma chamber has been totally replenished by the new basaltic magma, and seems a prelude to the progressive ceasing of a major eruptive cycle. 相似文献
6.
Petrogenesis and significance of the Mesozoic North Taihang complex: Major and trace element evidence 总被引：22，自引：1，他引：21
The intensive Mesozoic magmatism in the North China Craton (NCC) has drawn great attention for its particular geochemical signatures (e.g. high-K), petrogenesis and tectonic setting. The North Taihang complex represents the westernmost magmatic belt of th… 相似文献
7.
Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial 87Sr/86Sr (0.7063–0.7076) and low initial Nd isotope ratios (?Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial 87Sr/86Sr (usually less than 0.7060) and higher Nd isotope ratios (?Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone. 相似文献
8.
Zhang Guangliang 《中国地震研究》2006,20(2):216-224
INTRODUCTIONThe mafic-ultramafic complexesinthe Hongqilingarea were emplacedintothe metamorphic rocksof the Hulan Group. Age determination of the intrusion and metamorphism of the Hulan Groupmetamorphic rocks is crucial for the study of petrogenesis and evolution, orogenesis and itsdevelopment of the region. However ,so far it has been difficult to determine the geochronology ofmafic-ultramafic rocks inthe area ,thusthe age obtainedfromprevious data hadto be used.Inrecentyears ,withthe … 相似文献
9.
Numerical modeling of the formation of Indo-Sinian peraluminous granitoids in Hunan Province: Basaltic underplating versus tectonic thickening 总被引：12，自引：1，他引：12
The genesis of Indo-Sinian granitic plutons with peraluminous and potassium-rich affini-ties from Hunan Province, China has been investigated by numerical modeling using the numericalcode FLAC. On the basis of the regional geological evolution in South China, we employed a real-istic numerical model in an attempt to unravel the influences of basaltic underplating and tectoniccrustal thickening on the crustal anatexis. Heat production derived from basaltic underplating (e.g.ca. 220 Ma gabbro xenoliths) can result in dehydration melting of fluid-bearing minerals in crustalrocks such as gneisses and metapelites, but its effect is limited in a relatively short time span (5-15 Ma) and on a small scale, Accordingly, it is very difficult for basaltic underplating to generate thelarge-scale Indo-Sinian granitic bathliths unless voluminous mafic magmas had been underplatedat the lower/middle crust during this period. Alternatively, crustal thickening induced by tectoniccompression can also lead to geothermal elevation, during which the temperature at the boundarybetween lower and middle crusts can be up to or greater than 700℃, triggering dehydration melt-ing of muscovite in gneiss and metapelite. The proportion of melts from muscovite-induced dehy-dration melting is close to critical melt percentage (≥20%) once the thickening factor reaches 1.3.These melts can be effectively transferred to the crust-level magma chamber and form large-scalegranitic batholiths. In combination with the Indo-Sinian convergent tectonic setting in South Chinaas well as sparse outcrops of contemporary mafic igneous rocks, we consider that tectonic crustalthickening is likely to be the predominant factor controlling the formation of the Indo-Sinian per-aluminous and potassium-rich granitoids in Hunan Province. 相似文献
10.
J.T. Caulfield S.P. Turner A. Dosseto N.J. Pearson C. Beier 《Earth and Planetary Science Letters》2008,273(3-4):279-288
The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism.We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (< 17) with values as low as 7.2. Melting models show that large degree melts of depleted MORB mantle fail to reproduce the observed Nb/Ta. Alternatively, incorporation of residual back-arc mantle that has undergone less than 1% melting into the sub-arc melting regime reproduces arc values. However, the extent of partial melting required to produce the composition of the Lau Basin back-arc basalts averages 7%. This apparent discrepancy can be explained if only the lowermost 4 km of the residua from the mantle melt column beneath the back-arc is added to the source of arc magmas. We have identified that the degree of arc/back-arc coupling displayed in the rock record provides an index of the depth of hydrous melting beneath the arc. In this case, this would imply a depth of ~ 75 km for generation of arc magmas, indicating that hydrous melting in the mantle wedge is triggered by the breakdown of hydrous phases in the subducting slab. 相似文献
11.
Abstract The Mefjell plutonic complex consists of 500–550‐Ma Pan‐African plutonic rocks, which intrude into the Precambrian crystalline basement in the Sør Rondane Mountains, East Antarctica, and forms part of the Sør Rondane Suture Zone. The complex comprises syenitic and granitic (mostly monzogranitic) rocks, and is characterized by the presence of iron‐rich hydrous mafic minerals and primary ilmenite, both of which imply its formation at high temperature and under low oxygen fugacity conditions. The syenitic rocks are metaluminous, and are high in alkalis, K2O/Na2O, Al2O3, FeOt/(FeOt + MgO) (0.88–0.98), K/Rb (800–1000), Ga (18–28 p.p.m.), Zr (up to 2100 p.p.m.) and Ba. They also have a low Mg? (Mg/[Mg + Fe2+]), Rb, Sr, Nb, Y and F, low to moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios and positive Eu anomalies in their rare earth element (REE) patterns. The granitic rocks are metaluminous to peraluminous, and have a high Rb content, high Sr/Ba and LREE/HREE ratios, low K/Rb and negative Eu anomalies. Most of the syenitic and granitic rocks have Y/Nb ratios greater than 1.2, and are depleted in Nb, Ti and Sr on the primitive mantle‐normalized spider diagrams, indicating a crustal origin with subduction zone signatures. We interpret both the syenitic and granitic rocks to be derived from an iron‐rich lower crustal source by dehydration melting induced by the heat of mantle‐derived basaltic intrusion, after which they then underwent limited fractional crystallization. The Mefjell plutonic complex has a high Zr content and tectonic discrimination diagram signatures indicative of normal A‐type granitic rocks. Both rock suites may have been generated under the same postorogenic tectonic setting. The Mefjell syenitic rocks are chemically comparable to charnockites in the Gjelsvikjella and western Mühlig‐Hofmannfjella areas of East Antarctica, whereas the granitic rocks are comparable to aluminous A‐type granitic rocks in South India, which were emplaced during formation and evolution of the Gondwanaland supercontinent. 相似文献
12.
Calc-alkaline intermediate rocks are spatially and temporally associated with high-Mg andesites (HMAs, Mg#>60) in Middle Miocene Setouchi volcanic belt. The calc-alkaline rocks are characterized by higher Mg# (strongly calc-alkaline trend) than ordinary calc-alkaline rocks at equivalent silica contents. Phenocrysts in the intermediate rocks have petrographical features such as: (1) coexisting reversely and normally zoned orthopyroxene phenocrysts in single rock; (2) sieve type plagioclase in which cores are mantled by higher An%, melt inclusion-rich zone; and (3) reversely zoned amphibole phenocrysts with opacite cores. In addition, mingling textures and magmatic inclusions were observed in some rocks. These petrographic features and the mineral chemistry indicate that magma mixing was the most important process in producing the strongly calc-alkaline rocks. The core composition of normally zoned orthopyroxene phenocrysts and the mantle composition of reversely zoned orthopyroxene phenocrysts have relatively high Mg# (85–90) in maximum. Although basaltic and high-Mg andesitic magmas are candidate as possible mafic end-member magmas, basaltic magma is excluded in terms of phenocryst assemblage and bulk composition. HMA magmas are suitable mafic end-member magmas that precipitated high Mg# (90) orthopyroxene, whereas andesitic to dacitic magma are suitable felsic end-members. In contrast, it is difficult to produce the strongly calc-alkaline trend through fractional crystallization from a HMA magma, because it would require removal of plagioclase together with mafic minerals from the early stage of crystallization, whereas the precipitation of plagiolase is suppressed due to the high water content of HMA magmas. These results imply that Archean Mg#-rich TTGs (>45–55), which are an analog of the strongly calc-alkaline rocks in terms of chemistry and magma genesis, can be derived from magma mixing in which a HMA magma is the mafic end-member magma, rather than by fractional crystallization from a HMA magma. 相似文献
13.
The possible effect of pressure-induced breakdown of amphibole in triggering explosive eruptions is considered. Since amphibole is a hydrous mineral, when it breaks down to an anhydrous assemblage as pressure is reduced to less than 1.5–2 kbar, the water liberated might oversaturate the coexisting melt generating the necessary overpressure to trigger an explosive eruption. Resorbed amphiboles are commonly observed in evolved lavas and pyroclastic ejecta. The amount of a volatile component, such as water that will dissolve in a melt is a function of pressure, temperature and composition, and during crystallization it is also a function of the extent of crystallization and the nature of crystallizing minerals. The relation can be expressed by the simple equation:
where Xr is the water content of the residual liquid, Xi is the initial water content, XmOH, is the water content of hydrous minerals, f is the total extent of crystallization and f′ is the extent of crystallization of hydrous minerals such that 0 ≤ f′ ≤ f ≤ 1. We suggest that storage of water in hydrous minerals, such as amphibole and biotite, plays an important role in the eruptive behavior of certain types of magmas; the breakdown of these minerals liberates water to the melt at a rate governed by the kinetics of the resorption reaction. If the release of water causes the liquid fraction to exceed the solubility limit and the overpressure resulting from expansion of the gas exceeds the strength of the overlying magma and rocks in the conduit, the result can be an explosive eruption. The amphibole effect can occur at different structural levels depending on the nature of the magma and physical conditions leading to instability. 相似文献
14.
Age and generation of Fogang granite batholith and Wushi diorite-hornblende gabbro body 总被引：3，自引：0，他引：3
XU XiSheng? LU WeiMin & HE ZhenYu State Key Laboratory of Crust-Mantle Evolution Mineralization Department of Earth Sciences Nanjing University Nanjing China 《中国科学D辑(英文版)》2007,50(2):209-220
Fogang granitic batholith, the largest Late Mesozoic batholith in the Nanling region, has an exposure area of ca. 6000 km2. Wushi diorite-hornblende gabbro body is situated at the northeast part of the ba- tholith. Both the granitic batholith main body and the diorite-hornblende gabbro body belong to high-K calc alkaline series. Compared with the granitic main body, the Wushi body has lower Si (49%―55%), higher Fe, Mg, Ca, lower REE, less depletion of Eu, Ba, P, Ti, and obvious depletion of Zr, Hf. Zircon LA-ICP-MS dating and the mineral-whole rock isochron dating reveal that Fogang granitic main body and Wushi body were generated simultaneously at ca. 160 Ma. The Fogang granitic main body has high (87Sr/86Sr)i ratios (0.70871―0.71570) and low εNd(t) values (?5.11―?8.93), suggesting the origins of the granitic rocks from crustal materials. Their Nd two-stage model ages range from 1.37―1.68 Ga. The Sr and Nd isotopic compositions and the Nd model ages of the granitic rocks may suggest that the giant Fogang granitic main body was generated from a heterogeneous source, with participation of mantle component. Wushi diorite-hornblende gabbro is an unusual intermediate-basic magmatic rock series, with high (87Sr/86Sr)i ratios (0.71256―0.71318) and low εNd(t) values (?7.32―?7.92), which was possibly formed through mixing between the mantle-derived juvenile basaltic magma and the magma produced by the dehydration melting of lower crustal basaltic rocks. 相似文献
15.
The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO2 . They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K2 O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc. 相似文献
16.
We present the initial results of a quantitative investigation of the volatile geochemistry of Deception Island, an active volcano situated near the spreading axis of a Quaternary ensialic marginal basin (Bransfield Strait, northern Antarctic Peninsula). Fluorine contents in Deception Island magmas (112–461 ppm) are comparable with lavas from a range of tectonic environments but F-K2O relationships most closely compare with continental flood basalts and lavas from island arcs and some marginal basins. Boron contents are high (4.3–16.3 ppm) and the values overlap with those of arc lavas; they provide strong support for the presence of a mantle source component derived from the slab subducted at the coeval trench (by melting at the slab/wedge interface and/or during slab dehydration). Both F and B acted incompatibly in Deception Island magmas but there is significant variation in incompatible-element ratios such as K/F, K/B, P/F, P/B, which strongly suggests that the magmatic system was open to some or all of these elements during differentation. The variations in these ratios also provide evidence for the presence of at least two stages in the magmatic evolution of the volcano. During pre-caldera times, mafic magma was emplaced into the upper crust where it evolved and may have reacted with the crustal envelope, thus changing the contents of some or all of the elements F, B, K and P and their inter-element ratios. A later, large influx of hot, mafic magma into the chamber may have been responsible for a major eruption that ultimately led to the formation of the caldera. K/F, K/B, etc, ratios in the magma chamber were “reset” and subsequently continued to change, possibly by further crustal interaction during melt evolution in post-caldera times. 相似文献
17.
During the 1929 activity of Hokkaido-Komagatake volcano, the Plinian eruption of a phenocryst-rich andesite was preceded by a small eruption of more mafic magma formed by magma mixing. A similar eruption sequence has been reported for some other eruptions (Pallister et al. 1996; Venezky and Rutherford 1997), suggesting that eruption of a mixed magma is a precursor of phenocryst-rich magmas. For the purpose of understanding the tapping processes of the phenocryst-rich magma chamber, we investigated the temporal variation in the erupted magma and estimated the viscosity and density of the end-member and mixed magmas with constraints drawn from petrography. For the precursory mixed magma we estimate 33dž vol.% phenocrysts, andesitic-dacitic melt composition, 3 wt.% H2O content, and temperature of 1040°C. In comparison, for the climactic, silicic end-member magma we estimate 48Dž vol.% phenocryst, high-silica rhyolitic melt, 3 wt.% H2O, and temperature of 950°C, respectively. The mafic end-member magma, which was not erupted, is thought to be an almost aphyric basaltic-andesitic magma, based on mass balance calculation of the phenocryst content. The proportion of the mafic end-member magma component in the mixed magma was calculated to be 20-40 wt.%. On the basis of these data, we estimate magma viscosities of 103.9, 106.9, and 102.0 Pa s for the mixed, silicic end-member, and mafic end-member magmas, respectively. The calculated density differences among these magmas are inconsequential when possible errors are considered. We calculate the minimum excess pressure required for dike propagation to be 31 MPa for the silicic end-member magma and 8 MPa for the mixed magma, using the estimated viscosity and dike propagation model of Rubin (1995). If we assume that excess pressure is limited by the wall rock strength of the magma chamber, excess pressure retainable in the magma chamber is less than ca. 20 MPa. This suggests that the mixed magma was able to ascend to the surface without freezing, whereas the viscous silicic end-member magma could not. The formation and precursory eruption of the mixed magma are, therefore, effective and necessary initiation processes for the phenocryst-rich, viscous magma eruption. 相似文献
18.
The oxygen isotope systematics of Tertiary volcanic rocks of east-central Nevada and of plutonic and metamorphic rocks of the Ruby Mountains-East Humboldt Range core complex provide complementary evidence for major18O-depletion and 18O/16O homogenization of mid-crustal rocks during metamorphism and magmatism. The δ18O value of crustal source material for silicic volcanic rocks decreased from between +9 and +11‰ to between +7 and +8‰ over 5 Ma. Mid-crustal metasedimentary and granitic rocks in the East Humboldt Range have δ18O values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels. Exchange with deep-seated mantle-derived igneous rocks, or fluids derived therefrom, is the most plausible18O-depletion mechanism. Intrusion of these mafic magmas promoted crustal melting and fluid migration. Homogenization of 18O/16O resulted from migration of high-temperature fluids and melts at mid-crustal levels, and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks. 相似文献
19.
J. Klerkx S. Deutsch H. Pichler W. Zeil 《Journal of Volcanology and Geothermal Research》1977,2(1):49-71
The Cenozoic volcanic rocks of the southern Andes are characterized by low 87Sr/86Sr ratios (0.7040–0.7045), which are consistent with an origin in the downgoing slab of oceanic lithosphere or the overlying mantle. These values are distinctly lower than those from corresponding rocks of the central Andes.The calc-alkaline rocks of the central Andes exhibit higher Sr isotopic values (0.705–0.713) and variable Rb/Sr ratios. Different explanations are possible for this behaviour as well as for the positive correlation between 87Sr/86Sr and Rb/Sr expressed in an apparent isochron of 380 ± 50 m.y. It is postulated that these magmas result from a mixing process between a primary magma with basaltic affinities and crustal material of relatively young age.A model is proposed for the generation of the “andesitic” magmas of the central Andes by which crustal rocks of the upper part of the crust are added to the base of the crust by an accretionary process at the margin of the continent. Melts from these upper crustal rocks act as contaminants in “andesitic” magmas.The role of crustal material is still more significant in the generation of the ignimbritic magmas; they are considered to result from a two-stage melting process by which igneous rocks, belonging to a former stage of development of the Andes, are engulfed in the subduction zone, where they melt. 相似文献
20.
The application of trace elements to the petrogenesis of igneous rocks of granitic composition 总被引：2，自引：0，他引：2
Gilbert N. Hanson 《Earth and Planetary Science Letters》1978,38(1):26-43
Although trace element modeling has been used to great advantage for petrogenetic interpretations of basaltic systems, similar studies on igneous rocks of granitic composition have been fewer. In general the mineral/melt distribution coefficients for rare earth elements (REE) in granitic melts are equal to or greater than those for similar minerals in the basaltic system. Thus the effects of these minerals on the REE patterns of granitic melts during partial melting or differentiation are exaggerated as compared to basaltic systems, making detection of residual phases easier. For the K/Rb ratio, if neither a K-feldspar component nor biotitephlogopite is present in the residue, it is difficult to reduce the K/Rb ratio of the melt relative to the parent by a factor of two by either differentiation or partial melting.The petrogenesis of four distinctly different rocks are received: (1) an Archean tonalite presumably derived by partial melting of an Archean tholeiite at mantle depths, leaving a garnet plus clinopyroxene residue; (2) an Archean quartz monzonite presumably derived by partial melting of a short-lived graywacke-argillite sequence at crustal depths; (3) a dacite from Saipan presumably derived by differentiation from a basaltic parent; and (4) a trachyte from Ross Island, Antarctica, presumably derived by differentiation from a basanitoid parent and contaminated by continental crustal components. 相似文献