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1.
Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (87Sr/86Srinitial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy  相似文献   

2.
The Bingham porphyry Cu-Au-Mo deposit, Utah, may only be world-class because of substantial contributions of sulfur and metals from mafic alkaline magma to an otherwise unremarkable calc-alkaline system. Volcanic mafic alkaline rocks in the district are enriched in Cr, Ni, and Ba as well as Cu, Au, platinum group elements (PGE), and S. The bulk of the volcanic section that is co-magmatic with ore-related porphyries is dacitic to trachytic in composition, but has inherited the geochemical signature of high Cr, Ni, and Ba from magma mixing with the mafic alkaline rocks. The volcanic section that most closely correlates in time with ore-related porphyries is very heterogeneous containing clasts of scoriaceous latite, latitic, and minette, and flows of melanephelinite, shoshonite, and olivine latite in addition to volumetrically dominant dacite/trachyte. Bingham ore-related porphyries show ample evidence of prior mixing with mafic alkaline magmas. Intrusive porphyries that have not been previously well-studied have several chemical and mineralogical indications of magma mixing. These "mixed" lithologies include the hybrid quartz monzonite porphyry, biotite porphyry, and minette dikes. Even some of the more silicic latite and monzonite porphyries retain high Cr and Ba contents indicative of mixing and contain trace amounts of sapphire (<1 mm). The heterogeneous block and ash flow deposits also contain sapphire and are permissively correlated with the intrusions based on chemical, mineralogical, and isotopic data. Magma mixing calculations suggest about 10% of the monzonitic/latitic ore-related magma may have been derived from mafic alkaline magma similar to the melanephelinite. If the original S content of the mafic magma was about 2,000-4,000 ppm, comparable with similar magmas, then the mafic magma may have been responsible for contributing more than half of the S and a significant portion of the Cu, Au, and PGE in the Bingham deposit.  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.
Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO2 = 62.5 wt%) to latite (SiO2 ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO2 ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1). Received: 23 March 1998 / Accepted: 11 December 1998  相似文献   

5.
Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO2 ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al2O3, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K2O(4.9–5.5 wt.%), TiO2 (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low 143Nd/144Nd 0.512372–0.512536, middle 87Sr/86Sr 0.707322–0.707395, middle 206Pb/204Pb 18.50–18.59, 207Pb/204Pb 15.60–15.65 and 208Pb/204Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO2 content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median 87Sr/86Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or70Ab28An2) and 35% SiO2. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan.  相似文献   

6.
How late are K-feldspar megacrysts in granites?   总被引：1，自引：0，他引：1  
R.H. Vernon  S.R. Paterson 《Lithos》2008,104(1-4):327-336
Various petrologists have suggested that K-feldspar megacrysts grow in granites that are extensively crystallized, even at subsolidus conditions. However, experimental evidence indicates that, though K-feldspar nucleates relatively late in the crystallization history, abundant liquid is available for development of large crystals. A great deal of evidence, involving many different factors, favours a magmatic/phenocrystic origin for K-feldspar megacrysts in granites, namely simple twinning, oscillatory zoning, euhedral plagioclase inclusions, and concentric, crystallographically controlled arrangements of inclusions. In addition, abundant evidence has been presented of (1) mechanical accumulation of K-feldspar megacrysts in granites, (2) alignment of megacrysts and megacryst concentrations in magmatic flow foliations, (3) involvement of megacrysts in zones of magma mixing in granite plutons, and (4) occurrence of megacrysts in some volcanic rocks, implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation. Detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas. Irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased, the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains.  相似文献   

7.
The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene–plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)–troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ18O (+5 to +6‰), initial 87Sr/86Sr (Sri=0.7030 to 0.7054), Nd (+5 to −1) values whereas noritic cumulates are variably enriched in δ18O (+7 to +9‰), show negative Nd (−7 to −12) and slightly higher Sri (0.7040–0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sri=0.7030; Nd=+5.1; δ18O=+5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (<10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO2 activity, the enrichment in 18O and 87Sr and the lowering of Nd in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time.  相似文献   

8.
Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. 87Sr/86Sri and 143Nd/144Ndi isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (87Sr/86Sri=0.7047–0.7064, 143Nd/144Ndi=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, LaN up to 400). The isotopic ratios approach crustal values (87Sr/86Sri=0.7059–0.7115 and 143Nd/144Ndi=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).  相似文献   


9.
胶州大西庄玄武岩（73 Ma）中的辉石巨晶与寄主玄武岩具有几乎完全相同的同位素组成,其87Sr/86Sr（t）和εNd（t）分别为0.703 3～0.704 1、+4.5～+6.5和0.703 5～0.703 8、+7.1～+7.6,暗示辉石巨晶和寄主岩具有相同的地幔源区,即辉石巨晶是玄武岩浆在高压下结晶的产物.亏损的同位素特征表明：在白垩世末期（～73 Ma）,胶东地区的岩石圈地幔已经由富集转变为亏损,表现出类似于中国东部新生代地幔的同位素特征,即表明当时岩石圈减薄（或置换）已经完成.  相似文献   

10.
Ultramafic inclusions and megacrysts are unusually abundant in a nephelinite sill in the Nandewar Mountains in north-eastern New South Wales. The inclusions are divisible into a Cr-diopside group and a Ti-augite group, the former being dominated by Cr-spinel Iherzolites of restricted modal composition, the latter by olivine and titaniferous Al-rich clinopyroxene assemblages which vary widely in their modal proportions. The principal megacryst species are olivine and black, titaniferous Al-rich clinopyroxene; additional but comparatively rare megacrysts include titanphlogopite, kaersutitic amphibole, and deep green, relatively Fe-rich clinopyroxene. The Cr-spinel Iherzolites conform closely in mineralogy and chemistry with the spinel lherzolites which dominate upper mantle xenolith assemblages in alkaline mafic volcanic rocks from other provinces. Megacrysts and Ti-augite inclusion mineral assemblages are consistently more Fe-rich than analogous phases in the Cr-diopside xenoliths and also display more extensive cryptic variation. The available experimental data on the high pressure liquidus or near-liquidus phases in olivine nephelinite and related compositions indicate that the olivine and black clinopyroxene megacrysts were precipitated at pressures in the vicinity of 15–20 kb. The similarity in the nature and compositions of the principal megacryst species to analogous phases in the Tiaugite group of inclusions indicates that the latter also represent cognate cumulates derived from the olivine nephelinite at broadly comparable pressures. High pressure fractionation of the host olivine nephelinite liquid, controlled mainly by the separation of olivine and aluminous clinopyroxene, produced only comparatively minor compositional changes in the derivative liquid. The hiatus in olivine compositions at approximately Fo86–88, apparently characteristic of the olivines in coexisting Cr-diopside and Ti-augite inclusions, is assessed in terms of the compositions of olivine in equilibrium with alkali basaltic liquids at high pressures.  相似文献   

11.
Six volcanic rocks, reconnaissance samples representing most of the temporal and compositional variation in the Pinacate volcanic field of Sonora and Arizona, are characterized for major element and Nd---Sr isotopic compositions. The samples consist of basanite through trachyte of an early shield volcano, and alkali basalts and a tholeiite from later craters and cinder cones. With the exception of the trachyte sample, which has increased 87Sr/86Sr due to crustal effects, all 87Sr/86Sr values fall between 0.70312 and 0.70342, while εNd values are all between + 5.0 and + 5.7. Clinopyroxene in a rare spinel-lherzolite nodule derived from the uppermost mantle beneath the field has 87Sr/86Sr of 0.70320 but εNd of + 8.8, three εNd units higher than the volcanic rocks. Both the volcanic rocks and the nodule record the presence of asthenospheric, rather than enriched lithospheric mantle beneath Pinacate. This is consistent with one or both of (a) proximity of Pinacate to the Gulf of California spreading center and (b) presence of similar asthenospheric mantle signatures in volcanic rocks over a wide contiguous area of the southwestern USA. We consider the comparison to other southwestern USA magma sources as the more relevant alternative, although a definite conclusion is not possible at this stage.  相似文献   

12.
位于南秦岭武当地区的天宝铌矿床是我国典型的碱性火山岩型铌矿床,但其岩浆成因、演化及成矿机理等方面的研究十分薄弱.天宝碱性火山岩主要分为粗面质岩石和碱性玄武岩两大类,两者在空间上紧密共生.在详细的岩(矿)相学及榍石显微结构研究的基础上,利用LA-ICP-MS分析技术对天宝粗面质火山岩中的榍石进行原位U-Pb定年,并利用E...  相似文献   

13.
松辽盆地徐家围子断陷营城组火山岩中含有粗面质岩石,是深层天然气重要储层之一。岩心、钻井及地震资料研究表明,粗面岩具有高位喷发、低位充填的特征,在火山口附近厚度大,远离火山口厚度小。粗面岩主量、微量及同位素地球化学显示其属于钾玄岩系列,富集稀土元素,强不相容元素Rb、Ba、Th、U质量分数较高。粗面岩的(87Sr/86Sr)i=0.704 321~0.705 395,εNd(t)为正值（3.36 ~ 3.83）,Pb同位素比值相对集中,(206Pb/204Pb)i、(207Pb/204Pb)i、(208Pb/204Pb)i平均值分别为18.43、15.51和38.23。研究表明,粗面质岩浆由区域年轻地壳组分部分熔融形成,经历了一定程度的分离结晶作用,地壳混染作用不显著。粗面质岩浆多期次喷发后形成粗面岩,热液气体不断聚集发生隐蔽爆破形成角砾,未固结的角砾被岩浆期后热液“胶结”,形成隐爆角砾岩,构成了粗面质岩石重要的储层类型。  相似文献   

14.
北京燕山构造旋回安粗岩系   总被引：4，自引：0，他引：4  
安粗岩系是指由钾质粗面玄武岩，橄榄玄武粗安冉、安粗岩，粗面岩和高钾流纹岩等富钾岩石组成的火山岩系列。本文以北京地区燕山构造旋回安粗岩系火山岩为例，通过与国内外公认的安粗岩系比较，以及对不同系列火山岩特征的对比分析，明确提出：安粗岩系是可以与拉斑玄武岩系、钙碱性岩系和碱性玄武岩系并列存在的独立的火山岩系列，它有自己特写的产出环境，有别于其他岩系的岩石类型，岩石组合及物质成分等特点。  相似文献   

15.
西藏安多地区粗面岩的特征及其锆石SHRIMP U-Pb定年   总被引：3，自引：2，他引：1  
安多粗面岩位于班公湖-怒江缝合带中段,紧邻安多蛇绿岩带的北侧,出露面积约30km2,堆积厚度约1500m,属陆相火山岩。火山机构面貌保存较完整。岩石类型包括安粗岩、石英安粗岩、粗面岩及少量火山碎屑岩,以粗面岩为主,属钾玄岩系列。岩石地球化学和同位素示踪特征表明岩石属相对高压型的粗面岩,是与大陆碰撞后造山有关的钾质火山岩。岩浆主要来源于加厚的陆壳下部,但也有幔源组分的贡献。粗面岩中锆石SHRIMP U-Pb年龄为79.9Ma±1.9Ma,是粗面质岩浆上侵结晶的年龄,表明安多粗面岩的喷溢时代为晚白垩世, 也暗示班公湖-怒江洋盆安多段早白垩世晚期已经闭合,晚白垩世时期羌塘地块与拉萨地块已成为统一的陆块,并具有加厚的陆壳。  相似文献   

16.
An analcimite sill, which intrudes Carboniferous sedimentary rocks northwest of the township of Barraba in northeastern New South Wales, is exceedingly rich in ultramafic and mafic inclusions and also contains a varied megacryst assemblage. The majority of inclusions belong to an ultramafic-mafic granulite suite whose members generally contain a Cr-poor green spinel. Layering is preserved in many inclusions and their textures are appropriate to those arising from recrystallization at subsolidus temperatures. Ultramafic granulites of the Al-spinel suite are mainly pyroxenites, with rarer lherzolites, and mafic granulites usually consist of the assemblage plagioclasea-luminous pyroxenes-spinel. Ca-rich tschermakitic clinopyroxenes and coexisting aluminous Ca-poor orthopyroxenes define a trend of moderate iron enrichment. Spinels also display significant Fe2+ → Mg substitution. Plagioclase in some plagioclase-bearing pyroxenites and mafic granulites contains numerous rod-like inclusions of spinel, compositionally similar to the discrete spinels unassociated with plagioclase. The formation of spinel in plagioclase is believed to have resulted largely from the migration of (Mg, Fe2+) to Al-rich nucleation sites in the feldspar. Other inclusion types include Cr-spinel lherzolites —more Fe-rich than Cr-diopside lherzolite inclusions in alkaline volcanics — and rare wehrlite heteradcumulates, probably cognate with the host analcimite. The megacryst assemblage is dominated by anorthoclase megacrysts, which are accompanied, in order of decreasing abundance, by megacrysts of tschermakitic clinopyroxene, titanbiotite, kaersutite, and aluminous titanomagnetite. The Al-spinel mafic granulites have low Ti, K and P contents and their petrochemical affinities are high-alumina mafic alkaline to transitional. They compare closely in major and minor element chemistry with some ocean ridge basalts. The Al-spinel ultramafic-mafic inclusions suite is interpreted as the remnants of a layered ultramafic-mafic “pluton ” which initially crystallized at pressures in the vicinity of 10 kb and subsequently re-equilibrated at subsolidus temperatures (ca 950° C) and comparable pressures. The parent magma was K-poor, ol-normative subalkaline and its fractionation at moderate pressures, controlled mainly by olivine and subcalcic clinopyroxene, resulted in decreases in the derivative liquids in their saturation levels and ol contents, and increases in Al and Ca. These trends are reflected in the compositions of the mafic granulites. The pressure regime of megacryst formation apparently was greater than 10–12 kb i.e. the megacrysts precipitated before acquisition of xenoliths of the Al-spinel granulite suite by the analcimite host. Anorthoclase fractionation produced only limited compositional changes in the original alkali basaltic melt.  相似文献   

17.
The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km2 in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A1 (1800–1780 Ma), main anorthositic A2 (1760 Ma), early gabbroic G3 (between 1760 and 1758 Ma), late gabbroic G4 (1758 Ma), and a suite of dykes D5 (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A1 rocks have lower Sr (87Sr/86Sr(1760) = 0.70233–0.70288) and higher Nd (εNd(1760) = 1.6–0.9) isotopic composition. The most widespread A2 anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: 87Sr/86Sr(1760) = 0.70362, εNd(1760) varies from 0.2 to − 0.7. The G3 gabbro–norite has slightly lower εNd(1760) varying from − 0.7 to − 0.9. Finally, G4 gabbroic rocks show relatively high initial 87Sr/86Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd(1760) varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd(1760) values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. 87Sr/86Sr(1760) values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.  相似文献   

18.
攀西古裂谷内龙帚山玄武岩—碱玄响岩建造   总被引：5，自引：0，他引：5       下载免费PDF全文
在川、滇、黔三省晚二叠世峨眉山玄武岩省的中部，产于攀西古裂谷内的龙帚山火山岩建造自下而上有4个喷发旋回：拉斑玄武岩、碱玄响岩、钾粗面玄武岩以及拉斑玄武岩。上下两个玄武岩旋回的岩性与该裂谷带以外大面积展布的高钛玄武岩岩性相同；而其碱玄响岩旋回主要由响碱玄岩—碱玄响岩—响岩岩系组成，夹粗安岩和安粗岩层，此旋回厚达千米，类似于非洲肯尼亚裂谷带的高原溢流响岩岩系。龙帚山晚二叠世火山岩建造的层序和岩石组合不同于古裂谷带外正常的大陆板内暗色岩喷出相的层序和岩浆演化规律，可能是地幔热柱和裂谷背景下岩石圈拉张复合作用的结果。  相似文献   

19.
The evolution of a Pan‐African (c. 900–550 Ma) suite of composite dikes, with latite margins and rhyolite interiors, from southwest Jordan is discussed. The dikes cut the Neoproterozoic calc‐alkaline granitoids and high‐grade metamorphic rocks (c. 800–600 Ma) of the northern Arabian‐Nubian Shield in Jordan and have been dated by the Rb‐Sr isochron method at 566±7 Ma. The symmetrically distributed latite margins constitute less than one‐quarter of the whole dike thickness. The rhyolite intruded a median fracture within the latite, while the latter was still hot but completely solidified. The dikes are alkaline and bimodal in composition with a gap in SiO2 between 61 and 74 wt%. Both end members display similar chondrite‐normalized rare earth element patterns. The rhyolites display the compositional signature of A‐type granites. The (La/Lu)N values are 6.02 and 4.91 for latites and rhyolites, respectively, and the rhyolites show a pronounced negative Eu anomaly, in contrast to the slight negative Eu anomaly of the latites. The chemical variability (e.g. Zr/Y, Zr/Nb, K/Rb) within and between latites and rhyolites does not support a fractional crystallization relationship between the felsic and mafic members of the dikes. We interpret the magma genesis of the composite dikes as the result of intrusion of mantle‐derived mafic magma into the lower crust in an extensional tectonic regime. The mafic magma underwent extensive fractional crystallization, which supplied the necessary heat for melting of the lower crust. The products of the initial stages of partial melting (5–10%) mixed with the fractionating mafic magma and gave rise to the latite melts. Further partial melting of the lower crust (up to 30%) produced a felsic melt, which upon 50% fractional crystallization (hornblende 15%, biotite 5%, feldspars 60%, and quartz 20%) gave rise to the rhyolitic magma. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

20.
微区-微量样品Rb-Sr同位素分析技术及其应用前景   总被引：1，自引：0，他引：1  
利用微钻取样技术和微量样品Rb-Sr同位素分析方法,本文对出露在东秦岭造山带的中生代合峪花岗岩的自形钾长石巨晶进行微区-微量样品Rb-Sr同位素组成分析。分析结果表明,钾长石斑晶具有显著的Rb/Sr比值和Sr同位素组成变化,斑晶和基质钾长石均构成年龄为132~133Ma的Rb-Sr等时线,代表岩浆的后期冷却时代。钾长石晶体的初始87Sr/86Sr比值由边缘相到中心相没有明显的变化,代表花岗质岩浆结晶阶段的Sr同位素组成,暗示合峪花岗岩的钾长石巨晶为原生成因。以高空间分辨率为特征,微区取样技术已经广泛地应用在变质岩和深成岩浆岩的同位素年代学和成因研究。结合微量样品同位素分析技术,微区-微量样品Rb-Sr同位素方法有望在火山岩的成因和年代学方面得到应用。  相似文献   

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