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1.
Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg# clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing. The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano. The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites, such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central to quantifying the subcrustal mass fluxes beneath Popo. Received: 13 December 1999 / Accepted: 11 August 2000  相似文献   

2.
New chronological, geochemical, and isotopic data are reported for Triassic (219–236 Ma) adakite-magnesian andesite-Nb-enriched basaltic rock associations from the Tuotuohe area, central Qiangtang terrane. The adakites and magnesian andesites are characterized by high Sr/Y (25–45), La/Yb (14–42) and Na2O/K2O (12–49) ratios, high Al2O3 (15.34–18.28 wt%) and moderate to high Sr concentrations (220–498 ppm) and εND (t) (+0.86 to +1.21) values. Low enrichments of Th, Rb relative to Nb, and subequal normalized Nb and La contents, and enrichments of light rare earth elements combine to distinguish a group of Nb-enriched basaltic rocks (NEBs). They have positive εND (t) (+2.57 to +5.16) values. Positive correlations between Th, La and Nb and an absence of negative Nb anomalies on mantle normalized plots indicate the NEBs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids. A continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite. The spatial association of the NEBs with adakites and magnesian andesites define an “adakitic metasomatic volcanic series” recognized in many demonstrably subduction-related environments (e.g., Mindanao arc, Philippines; Kamchatka arc, Russia; and southern Baja California arc, Mexico). The age of the Touhuohe suite, and its correlation with Triassic NEB to the north indicates that volcanism derived from subduction-modified mantle was abundant prior to 220 Ma in the central Qiangtang terrane.  相似文献   

3.
Summary Mineral compositions in leucite-bearing and leucite-free rocks from Vico volcano are reported. FeO/MgO partitioning (Kdol/liq) between olivine and latite (0.14–0.22), and between olivine and trachyte (0.06–0.10) indicates a lack of equilibrium between mineral and host rock. This suggests that mingling and/or mixing between magmas was a leading process during magmatic differentiation. In addition, a phono-tephrite olivine population with high (0.84) and equilibrium (0.23–0.29) Kdol/liq values has been produced by the interaction of differently evolved magmas. Zoning in clinopyroxene and plagioclase from these rocks recorded the same processes. In addition, resorbed quartz xenocrysts with coronas of clinopyroxene microlites indicate that digestion of crustal rocks occurred during the residence of magma in a shallow level reservoir. Increasing Fe coupled with decreasing Ca in diopside crystals from some phonolites, together with the petrographic and trace element data, indicate that polybaric fractional crystallisation also may be involved in the genesis of magmas of the second period of Vico activity. Leucite-free trachybasalts erupted in a late stage contain highly forsteritic olivine phenocrysts (forsterite 84–88 mol.%) in-equilibrium (Kdol/liq = 0.24–0.35) with the host rock, which indicate that they did not suffer chemical modification at low pressure. Received November 28, 2000; revised version accepted September 27, 2001  相似文献   

4.
Primitive arc magmatism and mantle wedge processes are investigated through a petrologic and geochemical study of high-Mg# (Mg/Mg + Fe > 0.65) basalts, basaltic andesites and andesites from the Kurile-Kamchatka subduction system. Primitive andesitic samples are from the Shisheisky Complex, a field of Quaternary-age, monogenetic cones located in the Aleutian–Kamchatka junction, north of Shiveluch Volcano, the northernmost active composite volcano in Kamchatka. The Shisheisky lavas have Mg# of 0.66–0.73 at intermediate SiO2 (54–58 wt%) with low CaO (<8.8%), CaO/Al2O3 (<0.54), and relatively high Na2O (>3.0 wt%) and K2O (>1.0 wt%). Olivine phenocryst core compositions of Fo90 appear to be in equilibrium with whole-rock ‘melts’, consistent with the sparsely phyric nature of the lavas. Compared to the Shisheisky andesites, primitive basalts from the region (Kuriles, Tolbachik, Kharchinsky) have higher CaO (>9.9 wt%) and CaO/Al2O3 (>0.60), and lower whole-rock Na2O (<2.7 wt%) and K2O (<1.1 wt%) at similar Mg# (0.66–0.70). Olivine phenocrysts in basalts have in general, higher CaO and Mn/Fe and lower Ni and Ni/Mg at Fo88 compared to the andesites. The absence of plagioclase phenocrysts from the primitive andesitic lavas contrasts the plagioclase-phyric basalts, indicating relatively high pre-eruptive water contents for the primitive andesitic magmas compared to basalts. Estimated temperature and water contents for primitive basaltic andesites and andesites are 984–1,143°C and 4–7 wt% H2O. For primitive basalts they are 1,149–1,227°C and 2 wt% H2O. Petrographic and mineral compositions suggest that the primitive andesitic lavas were liquids in equilibrium with mantle peridotite and were not produced by mixing between basalts and felsic crustal melts, contamination by xenocrystic olivine, or crystal fractionation of basalt. Key geochemical features of the Shisheisky primitive lavas (high Ni/MgO, Na2O, Ni/Yb and Mg# at intermediate SiO2) combined with the location of the volcanic field above the edge of the subducting Pacific Plate support a genetic model that involves melting of eclogite or pyroxenite at or near the surface of the subducting plate, followed by interaction of that melt with hotter peridotite in the over-lying mantle wedge. The strongly calc-alkaline igneous series at Shiveluch Volcano is interpreted to result from the emplacement and evolution of primitive andesitic magmas similar to those that are present in nearby monogenetic cones of the Shisheisky Complex.  相似文献   

5.
The basic and ultrabasic alkaline rocks of western Makhtesh Ramon, Israel crop out in numerous lava flows and subvolcanic bodies. The rock suite is composed of tephrite, basanite, basanitic nephelinite, analcimite, olivine nephelinite, and melilite-olivine nephelinite and in many outcrops is represented by glass-bearing varieties. Melt and fluid inclusions have been studied in olivine, clinopyroxene, and plagioclase phenocrysts. The EP, SIMS and microthermometry methods were used for inclusion study. The geochemical data obtained on glasses of melt inclusions (major, REE, trace elements, volatiles) are compared with the data on whole-rock and groundmass glass compositions. The compositions of melt inclusions reflect the different stages of rock crystallization: the initial products of crystallization are similar to whole-rock compositions whereas final portions of melts are usually enriched in SiO2, Al2O3, and alkalis, and depleted in mafic components. The data on contemporaneous melt and CO2 inclusions were used for the evaluation of the P–T conditions of rock generation. The following parameters were obtained: tephrite: P = 6.3–7.7 kbar and T = 1,150–1,250°C; basanite: P = 6.6–9.2 kbar and T = 1,150–1,250°C; olivine and analcime-olivine nephelinite: P = 5.6–8.2 kbar and T = 1,150–1,250°C; melilite-olivine nephelinite: 4.0–5.4 kbar and T mainly between 1,150 and 1,200°C. Magma genesis was restricted to P–T conditions of spinel- and plagioclase-lherzolite fields. These data suggest the shallowest depth of magma genesis occurred in Makhtesh Ramon compared to other occurrences of Early Cretaceous magmatism at the Middle East. Differences in the degree of batch partial melting of the same source rocks best explain the diversity of the igneous suite in western Makhtesh Ramon.  相似文献   

6.
Melt inclusions in minerals from some volcanoes of the Kurile-Kamchatka region were examined.The studied basaltic andesites and andesites were sampled from volcanoes of the Central Kamchatka depression(Shiveluch and Bezymyannyi),Eastern Kamchatka volcanic belt(Avachinskii and Karymskii),and Iturup Island,Southern Kuriles(Kudryavyi).Basalts of the 1996 eruption of the Karymskii volcanic center and dacites of Dikii Greben'volcano,Southern Kamchatka were also studied.More than 260 melt inclusions from 31 rock samples were homogenized,and quenched glasses were analyzed using electron and ion microprobes.The compositions of melt inclusions in andesitic phenoerysts vary in silica contents from 56 to 80wt%.Al_2 O_3 ,FeO,MgO,CaO decrease and Na_2O and K_2O increase with increasing SiO_2.Many inclusions(about 80% )are dacitic or rhyolitic.However,the compositions of silicic glasses(>65wt% SiO_2)in andesites significantly differ in TiO2,FeO,MgO,CaO,and K_2O contents from those in dacites and rhyolites.High-potassium melts(K_2O 3.8～6.8wt% )with various SiO_2 from 51.4 to 77.2wt% were found in minerals of all volcanoes studied.This indicates a contribution of a component selectively enriched in potassium to magmas of the whole region.A great compositional diversity of melt inclusions in plagioelase phenocrysts from the Bezymyannyi andesites suggests a complex history of plagioclase crystallization and magma evolution in the andesite formation.Melts from different volcanoes strongly vary in volatile contents.The highest H_2O contents are found in the melts from Shiveluch(3.0～7.2wt%,4.7wt% on average)and Avachinskii (4.7～4.8wt%);while those are lower in melts of Kudryavyi(0.1～2.6wt% ),Dikii Greben'(0.4～1.8wt%),and Bezymyannyi (<1wt%).Chlorine contents are also variable.The lowest values are found in the Bezymyannyi melts(0.09wt% on average),the highest Cl contents are typical of melt inclusions in minerals from the Karymskii andesites(0.26wt% on average).The melts from Avachinskii,Dikii Greben',Kudryavyi,and Shiveluch show intermediate Cl contents(0.13～0.20wt% ).The pressure of 350～1600 bar determined by CO_2 fluid inclusions in plagioclase from the Shiveluch andesites suggests a magma chamber at a depth of 1.5～6 km. Concentrations of 17 elements were determined in glasses of melt inclusions in plagioclases from five volcanoes(Avachinskii, Bezymyannyi,Dikii Greben',Kudryavyi,and Shiveluch).The studied melts show similar trace-element patterns with Nb and Ti minima and B,K,Be,and Li maxima.The melts are close to typical island arc magmas by Sr/Y,La/Yb,K/Ti,and Ca/St ratios, and have some specific regional geochemical features.REE patterns sensitive to degree of magma differentiation indicate that Kudryavyi magmas are most primitive,while Shiveluch magmas are most evolved.  相似文献   

7.
Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves.The major element contents show an obvious bimodal and trondhjemitic series evolutional trend.This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt.The contents of Rb,Sr and Ba are 7-21 ppm,153-363ppm and 201-1451 ppm respectively ,close to those of common Archaean grey gneisses.All the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6).The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks,belonging to typical TTG of Archaean high-grade metamorphic terrain .The gneiss is quite similar to the B-type Amitsoq gneiss of W.Greenland .The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust.  相似文献   

8.
Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO2), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60–62% SiO2, <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase phenocrysts are absent, and evenly distributed but sparse (0.5–3.5%) quartz xenocrysts are present in all the lavas. In order to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages (either olivine + augite or opx + augite) are temperatures in excess of 1000 °C, with water saturated liquids (>3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have ∼2 wt% lower Al2O3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al2O3 concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators of mantle melting processes. Received: 29 July 1997 / Accepted: 29 January 1998  相似文献   

9.
Mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35% olivine, occur at three different levels in Olivine-Bearing zones III and IV of the Middle Banded series of the Stillwater complex. The discordant bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses. Although the host olivine-gabbros are foliated with tabular plagioclase, the discordant bodies lack a discernible fabric and have blocky plagioclase. Average olivine in the host rocks is slightly more magnesian than that of the discordant bodies (Mg#75.8 ± 0.7 versus Mg#74.6 ± 0.3 respectively) but plagioclase compositions are indistinguishable (An77.6 ± 2.0 versus An76.6 ± 4.3– average host and discordant bodies respectively). Whole-rock major- and trace-element compositions of the discordant bodies are generally indistinguishable from cumulates with similar modal abundance. However, bulk compositions of anorthositic cores from the discordant bodies are enriched in K, Na, Ba, Sr and P. We conclude that the discordant bodies formed when cooler volatile fluids or fluid-rich silicate liquids moved upward and encountered a hotter undersaturated solid-plus-liquid assemblage. Continued liquid/fluid fluxing increased the permeability along the flow path and focused the flow, allowing the original bulk compositions to be modified and leaving plagioclase-rich troctolites and anorthosites. The shapes of the discordant bodies suggest that the cumulus pile had anisotropic permeability during late-stage liquid/fluid flow. Chemical and mineralogical evidence from other parts of Olivine-Bearing zones III and IV suggests that the processes that formed the discordant bodies may have influenced other cumulates. In fact, it appears that the same processes that formed the discordant bodies operated within an anorthositic layer, strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence. Received: 10 December 1996 / Accepted: 12 August 1997  相似文献   

10.
Dikii Greben' Volcano is the largest modern volcano with silicic rocks in the Kurile-Kamchatka island arc. It consists of many domes and lava flows of rhyodacite, dacite and andesite which were erupted in a reverse differentiation sequence. Non-equilibrium phenocryst assemblages (quartz + Mg-rich olivine, An-rich + An-poor plagioclase etc.), abundance of chilled mafic pillows in the dacites and andesites, and linear variations of rock compositions in binary plots are considered as mineralogical, textural and geochemical evidence for mixing. Mafic pillows in volcanics have a lower density (because of high porosity) and contain the same non-equilibrium phenocryst assemblages as the host rocks. Their groundmass contains skeletal microlites of plagioclase and amphibole proving that the groundmass as well as the pillows themselves formed from a water-rich basaltic magma at depth. They are considered as supercooled, vesiculated floating drops of a hot hybrid layer in the magma chamber which formed after refilling. The lower density of the inclusions allows them to float in the host magma and to concentrate at the top of the chamber prior to eruption. Magma mingling was effected by mechanical disintegration of the inclusions in the host magma during eruption. The rhyodacitic and basic end-members of the mixing series cannot be linked by low-P fractionation though high-P, amphibole-rich fractionation is not excluded.  相似文献   

11.
 Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48–52% SiO2) and minettes (49–54% SiO2) through basaltic andesites (53–57% SiO2), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO2); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (<550 ppm) and Ni (<450 ppm) accompanied by large concentrations of K, P, Ba (<4000 ppm) and Sr (<5000 ppm) and elements such as LREE and Zr (<600 ppm). No combination of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and Fe2O3 of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite would have a higher Mg# than one from olivine (Fo90) and pyroxene alone. As the Mascota series shows a correlation between K2O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite) in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from their mantle sources. Received: 24 January 1995 / Accepted: 21 February 1996  相似文献   

12.
Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province (EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG) sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector. A three-tier classification scheme has been adopted for the characterization and classification of individual lava flows. Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ). The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively. The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing. The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range ∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes. Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite component. Pyroxene phenocrysts are characterized by a prominent Ti-enrichment. Phenocrystal plagioclase grains are calcic (An52.7–An72.9), whereas groundmass plagioclase are relatively sodic (An39.2–An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals a temperature span of 850°C to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range from 1040°–1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption.  相似文献   

13.
In western part of the CEIM (Central-East Iranian Microcontinent) (Bayazeh area, Isfahan province, Iran), a series of Paleozoic basaltic rocks, occur. Major minerals of these basalts are olivine, clinopyroxene (diopside, augite), plagioclase (albite), sanidine, amphibole (kaersutite), phlogopite, ilmenite and magnetite. Secondary minerals include epidote, pumpellyite, albite, calcite and chlorite. Olivine and clinopyroxene are as phenocryst, while feldspars are restricted to groundmass. Chemical composition of clinopyroxenes indicates crystallization during ascending of magma. Geochemical analysis of whole rock samples shows that these rocks are characterized by low SiO2 (43.21–48.45 wt %), high TiO2 (1.81–3.00 wt %) and P2O5 (0.18–0.34 wt %). Petrography, chemistry of clinopyroxenes and whole rock analyses reveal an alkaline nature of these basalts. They are enriched in alkalis (Na2O + K2O = 4.1–7.7 wt %), LILE, HFSE and LREE. The Bayazeh alkali-basalts present strong enrichment in LREE relative to HREE (La/Lu ratio = 77.6–119.6) and were dominantly derived from partial melting of a metasomatized asthenospheric garnet-amphibole lherzolite. Field relationships reveal that junction of faults in west of the Bayazeh prepared a suitable path for ascending of magma from deep regions to surface and intra-plate continental magmatism. The Paleo-Tethys subduction from lower to upper Paleozoic is too enough for mantle enrichment in volatiles and basaltic alkaline magmatisrn in upper Paleozoic of Bayazeh area.  相似文献   

14.
A thick sequence of dark-red lava flows and clastic rocks with abundant volcanic debris crops out along the Río Wampú of eastern Honduras. Lithologic characteristics of these rocks, field interpretations of the Río Wampú stratigraphy, and limited K---Ar age determinations on volcanic samples indicate that these redbeds and associated volcanic units are correlative with the Upper Cretaceous Valle de Angeles Group of central Honduras. These strata uncomformably overlie dark clastic units which are tentatively correlated with the Jurassic Agua Fría Formation of the Honduras Group. The lava samples are predominantly microporphyritic andesites and basaltic andesites with abundant fine-grained phenocrysts of plagioclase, clinopyroxene and Fe---Ti oxides. Immobile-element whole-rock and mineral concentrations indicate a high-K tholeiitic composition. Elevated TiO2 whole-rock and clinopyroxene concentrations found in the basaltic andesites apparently reflect a primary magmatic characteristic. Based on geochemical and modal data, it appears that these rocks were erupted from partial mantle melts which were modified by substantial fractionation of modal phases. The presence of widespread Mesozoic redbeds and associated volcanics in Honduras was previously cited as evidence of regional extensional tectonics. However, structural, geochemical and stratigraphic data collected during this study indicate that the Río Wampú volcanic suite was formed during a compressional (subduction-related) event. Similar high-TiO2 basaltic andesites and andesites are reported along several active continental margins.  相似文献   

15.
Between 1953 and 1974, approximately 0.5 km3 of andesite and dacite erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest (or New) Trident. Field, analytical, and experimental evidence shows that the eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. Four lava flows (58.3–65.5 wt% SiO2) are the dominant products of the eruption; these contain discrete andesitic enclaves (55.8–58.9 wt% SiO2) as well as micro- and macro-scale compositional banding. Tephra from the eruption spans the same compositional range as lava flows; however, andesite scoria (56–58.1 wt% SiO2) is more abundant relative to dacite tephra, and is the explosively erupted counterpart to andesite enclaves. Fe–Ti oxide pairs from andesite scoria show a limited temperature range, clustered around 1000 °C. Temperatures from grains found in dacite lavas possess a wider range; however, cores from large (>100 μm) magnetite and coexisting ilmenite give temperatures of ∼890 °C, taken to represent a pre-mixing temperature for the dacite. Water contents from dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ∼3.5 wt% H2O, equivalent to 3 km depth if saturated. Unzoned pyroxene and sodic plagioclase in the dacite suggest that it likely underwent significant crystallization at this depth; highly resorbed anorthitic plagioclase from the andesite suggests that it originated at greater depths and underwent relatively rapid ascent until it reached 3 km, mixed with dacite, and erupted. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of earliest lava by approximately one month. The lack of a compositional gap in the erupted rock suite indicates that thorough mixing of the andesite and dacite occurred quickly, via disaggregation of enclaves, phenocryst transfer from one magma to another, and direct mixing of compositionally distinct melt phases. Received: 22 September 1999 / Accepted: 4 April 2000  相似文献   

16.
A spatial and temporal association between adakitic rocks and Nb-enriched basalts (NEB) is recognised for the first time in the western sector of the Trans-Mexican Volcanic Belt in the San Pedro–Cerro Grande Volcanic Complex (SCVC). The SCVC is composed of subalkalic intermediate to felsic rocks, spanning in composition from high-silica andesites to rhyolites, and by the young transitional hawaiite and mugearite lavas of Amado Nervo shield volcano. Intermediate to felsic rocks of the SCVC show many geochemical characteristics of typical adakites, such as high Sr/Y ratios (up to 180) and low Y (<18 ppm) and Yb contents. Mafic Amado Nervo rocks have high TiO2 (1.5–2.3 wt%), Nb (14–27 ppm), Nb/La (0.5–0.9) and high absolute abundances of HFSE similar to those shown by NEB. However, the Sr and Nd isotopic signature of SCVC rocks is different from that shown by typical adakites and NEB. Although the adakites–NEB association has been traditionally considered as a strong evidence of slab-melting, we suggest that other processes can lead to its generation. Here, we show that parental magmas of adakitic rocks of the SCVC derive their adakitic characteristic from high-pressure crystal fractionation processes of garnet, amphibole and pyroxene of a normal arc basalt. On the other hand, Amado Nervo Na-alkaline parental magmas have been generated by sediment melting plus MORB-fluid flux melting of a heterogeneous mantle wedge, consisting of a mixture of depleted and an enriched mantle sources (90DM + 10EM). We cannot exclude a contribution to the subduction component of slab melts, because the component signature is dominated by sediment melt, but we argue that caution is needed in interpreting the adakites–NEB association in a genetic sense.  相似文献   

17.
华北克拉通古元古代中期构造环境及演化至今尚存有争议,该时期相关岩浆作用的成因研究可为这些问题的解决提供重要证据。本文对华北克拉通中部山西吕梁地区古元古代中期形成的吕梁群近周营组基性火山岩开展综合研究,获得2 180±19 Ma 的形成年龄,岩石地球化学低TiO2、高Al2O3 和K2O,属钙碱性系列。微量元素富集Rb、Ba、Th、U、Sr 等大离子亲石元素,亏损Nb、Ta、Ti 和P等高场强元素,富集Pb,具轻稀土略富集、铕弱异常的右倾稀土模式,与典型消减带弧岩浆产物特征一致。同位素组成显示正εNd（t）= 1.0～2.6 和εHf（t）= 1.8～9.2, 指示它们来源于亏损地幔,低的Ti/Y（246～385）和Ce/Y 值（0.99～1.77）进一步揭示,地幔源区受到俯冲流体的交代和沉积物的参与,是楔尖晶石二辉橄榄岩与石榴石二辉橄榄岩过渡带略亏损地幔区发生部分熔融,并经历了橄榄石、单斜辉石等矿物分离结晶作用而形成。结合同期埃达克岩-高镁安山岩-富Nb玄武岩-安山岩组合以及弧花岗岩类的出现提出,2 200 Ma 华北中部带已演化至活动大陆边缘弧的构造环境。  相似文献   

18.
We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions. Received: 10 December 1996 / Accepted: 21 August 1997  相似文献   

19.
 All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49–54°S) have erupted exclusively adakitic andesites and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively thin crust (<35 km) within which plagioclase rather than garnet is stable. The source for AVZ adakites is more likely to be subducted oceanic basalt, recrystallized to garnet-amphibolite or eclogite. Geothermal models indicate that partial melting of the subducted oceanic crust is probable below the Austral Andes due to the slow subduction rate (2 cm/year) and the young age (<24 Ma) of the subducted oceanic lithosphere. Geochemical models for AVZ adakites are also consistent with a large material contribution from subducted oceanic crust (35–90% slab-derived mass), including sediment (up to 4% sediment-derived mass, representing approximately 15% of all sediment subducted). Variable isotopic and trace-element ratios observed for AVZ adakites, which span the range reported for adakites world-wide, require multistage models involving melting of different proportions of subducted basalt and sediment, as well as an important material contribution from both the overlying mantle wedge (10–50% mass contribution) and continental crust (0–30% mass contribution). Andesites from Cook Island volcano, located in the southernmost AVZ (54°S) where subduction is more oblique, have MORB-like Sr, Nd, Pb and O isotopic composition and trace-element ratios. These can be modeled by small degrees (2–4%) of partial melting of eclogitic MORB, yielding a tonalitic parent (intermediate SiO2, CaO/Na2O>1), followed by limited interaction of this melt with the overlying mantle (≥90% MORB melt, ≤10% mantle), but only very little (≤1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of Burney (52°S), Reclus (51°S) and northernmost AVZ (49–50°S) andesites and dacites require melting of a mixture of MORB and subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant northwards in the AVZ as the angle of convergence becomes more orthogonal. Received: 1 March 1995 / Accepted: 13 September 1995  相似文献   

20.
牛鼻子梁镁铁质-超镁铁质杂岩体岩石特征   总被引：1，自引：1，他引：0  
牛鼻子梁岩体位于柴达木地块的北缘,出露面积约8 km2,平面形态呈长条状,主要由斜长二辉橄榄岩、斜长单辉橄榄岩、角闪二辉橄榄岩、角闪橄榄岩、角闪橄榄二辉岩、黑云母化二辉岩、角闪辉石岩、橄榄辉石角闪石岩、角闪橄榄辉长岩、细粒辉长岩、似斑状辉长岩、暗色辉长岩、辉长岩、淡色辉长岩、石英闪长岩和英云闪长岩组成。文章通过岩石学、矿物学、地球化学研究,得到锆石U-Pb年龄为（361.5±1.2） Ma,Sm-Nd等时线年龄为（347±26） Ma。研究认为,牛鼻子梁基性-超基性岩体含矿岩石产于大陆边缘环境。岩体形成于泥盆纪晚期。岩浆分异充分,岩石类型丰富,岩浆演化过程中主要发生了橄榄石和斜长石的分离结晶/堆晶作用。岩体的母岩浆应属于拉斑玄武岩质岩浆。从目前发现的矿化情况来看,牛鼻子梁基性-超基性杂岩体为含矿岩体,有很好的找矿前景。  相似文献   

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