Garnet-bearing basalts: an example from Mt. Hasan, Central Anatolia, Turkey

Summary Mt. Hasan is a complex stratovolcano located in Central Anatolia, Turkey. It exhibits four evolutionary stages: (1) Kecikalesi volcano, (2) Palaeovolcano, (3) Mesovolcano and (4) Neovolcano. Each stage is characterized by differentiated magmas varying from basaltic andesite to rhyolite, but basaltic activities are recorded on the volcano flanks. Garnets occur in rhyodacitic and rhyolitic pumices of the Palaeo-, Meso- and Neovolcanoe, as well as in the basaltic andesite lava flows of Palaeovolcano. In addition, garnets are recognised also in recent basalts which were erupted contemporaneously with Neovolcano during the late Quaternary. Garnets appear as euhedral grains in a basaltic groundmass. Mineralogy and geochemistry of older volcanics (Kecikalesi, Palaeovolcano) exhibit a tholeitic trend, while Mesovolcano and Neovolcano are calc-alkaline in affinity. However, the recent basalts exhibit alkaline chemistry and mineralogy. Garnets of Mt. Hasan volcanic rocks are pyrope- and almandine-rich and display rather uniform composition throughout the volcanic evolution. Trace element data suggest that while subduction components in magma composition decreased from Miocene to late Quaternary, the within plate character increased. We propose that the generation of garnet-bearing volcanics reflects an enriched source inherited from ancient subduction processes. Received November 1, 1999; revised version accepted June 30, 2001     

Geochemistry of Archean Metavolcanic Rocks from Kadiri Schist Belt, Andhra Pradesh, India

The basic volcanic group exposed in the Kadiri schist belt includes high Mg-basalt, basalt, basaltic andestite and dacite. The basalts are tholeiitic in composition and high Mg-basalts, basaltic andesites and dacites show calc-alkaline affinity. Major and trace element characteristics suggest that the volcanic suite has been derived from an initial tholeiitic magma which has given rise to an early basaltic type and a later calc-alkaline type of rocks. An island arc and active continental margin tectonic setting was inferred for these rocks.     

Palaeoproterozoic subduction‐related and passive margin basalts in the Halls Creek Orogen,northwest Australia

Palaeoproterozoic basaltic rocks in the Halls Creek Orogen form part of two stratigraphic sequences on either side of a major structure, the Angelo‐Halls Creek‐Osmond Fault System. The two sequences have contrasting geological histories and probably formed in different tectonostratigraphic terranes. To the east, basalts of the Biscay Formation, which are part of the Halls Creek Group, were erupted at ca 1880 Ma and deformed and metamorphosed first at low grade between ca 1845 and ca 1820 Ma. To the west, basalts of the Tickalara Metamorphics were deposited after 1865 Ma, and were metamorphosed at medium to high grade and intruded by tonalite and leucogranite sheets at 1850 to 1845 Ma. Two groups of metabasalts are identified in the Biscay Formation. Group 1 samples have compositions similar to enriched (E‐)MORB. Group 2 samples have lower TiO₂, P₂O₅, Cr, Y, Nb and Zr contents, and trace‐element ratios (e.g. Ti/V and Zr/Nb), similar to low‐TiO₂ continental flood basalts. Metabasalts from the Tickalara Metamorphics consist of depleted and enriched types. The depleted samples have high field strength element (HFSE) and rare‐earth element (REE) abundances similar to oceanic island arc/backarc basin tholeiites. The enriched samples have compositions similar to E‐MORB, and are similarto group 1 samples from the Biscay Formation. Basalts of the Biscay Formation were erupted on a passive continental margin, whereas those of the Tickalara Metamorphics formed in an oceanic island arc/backarc basin or ensialic marginal basin, the two terranes being brought together by ca 1820 Ma. This is consistent with the evolution of the Halls Creek Orogen during the Palaeoproterozoic by plate‐tectonic processes similar to those operating in the Phanerozoic.     

Origin of the Felsic and Basaltic Dikes and Flows in the Rajula-Palitana-Sihor Area of the Deccan Traps,Saurashtra, India: A Geochemical and Geochronological Study

Rhyolite, trachyte, pitchstone, and granophyre dikes are associated with mafic dolerite dikes and basaltic flows of the northwestern part of the Deccan flood basalt province in the Saurashtra Peninsula, India. Felsic dikes, exposed in the Rajula area of Saurashtra, are similar in age to the basaltic flows of neighboring Palitana. The ages of both the felsic and mafic rocks straddle the ~65 Ma Cretaceous-Tertiary boundary and correspond to the main Deccan flood basalt episode. Palitana is centered on an elongated gravity high whose major axis is NE-SW, and Rajula is located on its southwestern flank. Unlike the younger Bombay felsic rocks from the western coast of India, which have been explained as partial melts of gabbros in deep crustal sills or previously erupted basalts, the incompatible-element characteristics of the Rajula rocks indicate that the Rajula rhyolites, trachytes, and dacites may have been generated by an almost complete melting of upper crustal rocks at the southwestern flank of the Rajula-Palitana-Sihor magmatic body. High potential temperatures of the Deccan plume, quick migration of the hot basaltic parent magma through lithospheric weak trends, and collection and residence of magma in upper-crustal magma chambers before eruption may have produced the right conditions to melt the upper crust in the vicinity of the Rajula-Palitana-Sihor magma chamber. On the other hand, the andesite located northeast of the magmatic body possibly evolved by assimilation of upper-crustal wall rocks accompanied by 5-10% crystallization of a Rajula-type basalt near the wall of the magma chamber. The Sihor rhyolites may also have been derived from the Sihor basalts through fractional crystallization accompanied by crustal assimilation. The Rajula granophyres, however, do not show any involvement of the upper crust in their genesis. These may have a history similar to that of the Bombay rocks and may have erupted in response to rifting along the Cambay rift.     

Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental Tisza–Dacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51–l71% SiO₂; mg-number0.65–0.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 5–35% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower ¹⁴⁹ Nd/¹⁴⁴ Nd and lower ²⁰⁶ Pb/²⁰⁴ Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination     

Geochemical constraints on the genesis of the volcanic rocks in the southeast of Isfahan area, Iran

Late Miocene–Pliocene to Quaternary calc-alkaline lava flows and domes are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in the Central Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and meta-aluminous. Major element variations are typical for calc-alkaline rocks. The volcanic rocks have SiO₂ contents ranging between 53.8% and 65.3%. Harker diagrams clearly show that the dacitic rocks did not form from the basaltic andesites by normal differentiation processes. They show large ion lithophile elements- and light rare earth elements (LREE)-enriched normalized multielement patterns and negative Nb, Ti, Ta, and P. Condrite-normalized REE patterns display a steep decrease from LREE to light rare earth elements without any Eu anomaly. These characteristics are consistent with ratios obtained from subduction-related volcanic rocks and in collision setting. The melting of a heterogeneous source is possible mechanism for their magma genesis, which was enriched in incompatible elements situated at the upper continental lithospheric mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved by contamination of a parental magma derived from metasomatized upper lithospheric mantle and crustal melts.     

Petrography and petrochemistry of latest Pliocene olivine-tholeiites of Taos area,northern New Mexico,U.S.A.

In north-central New Mexico, U.S.A., enormous volumes of basaltic rocks erupted in latest Pliocene time, forming large flat plateaus along the western foot of the Rocky Mountain Range.Petrographically, and also geologically, the volcanic rocks are divided into three series. Of these, the lava flows of the middle series accounted for more than ninety-five per cent of both area and volume. The first two series are composed of olivine-tholeiite while the last one is composed of calc-alkaline andesite.Twenty volcanic rocks, five groundmass plagioclases and five groundmass clinopyroxenes have been chemically analysed.The basalts are intermediate between tholeiitic and alkaline types and are rather similar to the high-alumina basalt from Oregon and the Cascade Range. However, the crystallization trends of groundmass plagioclase and clinopyroxene closely resemble those found in Japanese and Kilauean tholeiitic series.     

“三江”义敦岛弧带玄武岩喷发序列与裂谷—岛弧转化

岩石-构造组合是恢复古板块构造历史的最有效手段之一,同时是表征古板块边界与板内环境的最重要的地质证据。本文拟从岩石-构造组合角度,通过对义敦岛弧带玄武岩,特别是前岛弧期玄武岩喷发序列、岩石组合、地球化学特征和其形成背景的研究,试图从较深层次上揭示岩浆-构造内在联系,探索义敦古岛弧的形成与发展。     

Petrology and Geochemistry of the Rhobell Volcanic Complex: Amphibole-Dominated Fractionation at an Early Ordovician Arc Volcano in North Wales

The Rhobell Volcanic Complex is a remnant of a late Tremadoc,dominantly calc-alkaline, arc volcano. It is the only substantiallypreserved representative in the southern British Caledonidesof an early phase of Ordovician ensialic arc volcanism whichfollowed the onset of southeasterly subduction of Iapetus oceaniclithosphere beneath the northern margin of Gondwanaland. Thecomplex includes extrusive basalts and associated breccias,known as the Rhobell Volcanic Group, which rest unconformablyon folded, uplifted and eroded rift-basin sediments of the earlierpassive margin of the Iapetus Ocean. Amongst the basalts arerelatively primitive pargasite-bearing varieties which containcognate cumulate blocks, dominantly of pargasite but also withcalcic clinopyroxene, Ti-magnetite, and (rarely) plagioclase.Basaltic rocks also occur in an associated feeder-sheet intrusioncomplex, and as numerous minor sills and dykes. In the intrusioncomplex, basaltic sheets are cut by microdiorites and scarcemicrotonalites. The compositional range in the volcanic complex,from low-SiO₂ basalts to microtonalites (SiO₂ 45–66 wt.per cent), is attributable to fractional crystallization, earlystages of which were dominated by removal of pargasite at (water-undersaturated)pressures close to 10 kb, within the mantle. The parental magmawas derived by hydrous partial melting of a supra-subductionzone mantle wedge. Trace-element patterns indicate that themantle was slightly depleted relative to the putative primordialcomposition (Ta/Yb = 0?1), prior to metasomatism by componentsfrom the subduction zone. Textural variations in cumulate blocks,and various phenocryst forms in basalts, are interpreted asindicating that the erupted magmas came from a thermally andcompositionally stratified magma chamber with associated layeredcrystal accumulations, and that materials from initially separatelayers were mixed prior to eruption.     

Rare-earth elements in high-alumina basaltic rocks from Sardinia

High-alumina basalts and basic andesites, which represent the most “primitive” magma types of the Cenozoic andesitic series of Sardinia, show a spatial chemical zonation with respect to REE. The basaltic rocks from the northern and south-central part of the island have REE patterns typical of calc-alkaline rocks with an enrichment of light REE and fractionation of heavy REE. In contrast, those from the southernmost part have a pattern similar to typical continental tholeiites with only a small light-REE enrichment and unfractionated heavy REE.The present data suggest that basaltic rocks may be formed by anatexis of upper-mantle material with mineral assemblages containing either garnet (calc-alkaline rocks) or spinel (rocks of tholeiitic affinities). The presence of garnet or spinel could merely reflect mineral phase transformation and indicates a different depth of fusion for the various types of basaltic rocks with those of tholeiitic affinities originating at a shallower depth than the calcalkaline rocks. The REE data are consistent with the generation of the basaltic rocks by partial melting of mantle peridotite overlying a subducted plate.     

新疆甜水海地块种羊场石炭纪火山岩年代学和地球化学:岩石成因和地质意义

甜水海地块西段的种羊场地区发育一套互层状产出的玄武岩-玄武安山岩-流纹岩,本文对其进行了岩石学、同位素年代学和地球化学研究。结果表明,流纹岩LA-ICP-MS锆石U-Pb定年获得三组年龄:343.5±4.1Ma表明火山岩的形成时代为早石炭纪,2439±26Ma和1988±36Ma说明甜水海地块存在前寒武纪结晶基底。其中玄武质岩石岩性从拉斑系列、钙碱性系列向碱性系列过渡,呈现出E-MORB(OIB)、大陆板内拉张和岛弧的混合特征,与典型弧后盆地Okinawa玄武岩有一定的差异,表明其可能是异常陆缘弧后盆地拉张裂解的产物。玄武质岩石和流纹岩的主量元素、稀土元素和微量元素比值对的差异表明它们不是同源岩浆演化的产物,玄武质岩石的源区为类似E-MORB(OIB)的岩石圈地幔,且发生了部分熔融,原始岩浆上升过程中经历了矿物分离结晶和地壳混染作用。流纹岩属于高硅高碱的钙碱性火山岩,是上地壳部分熔融的产物。种羊场早石炭纪火山岩可能代表了古特提洋西端早期扩张的记录,为西昆仑-喀喇昆仑地区晚古生代多岛洋格局提供了新的证据。     

Calc-alkaline, Shoshonitic, and Primitive Tholeiitic Lavas from Monogenetic Volcanoes near Crater Lake, Oregon

Quaternary monogenetic volcanism in the High Cascades of Oregonis manifested by cinder cones, lava fields, and small shields.Near Crater Lake caldera, monogenetic lava compositions include:low-K (as low as 0?09% K₂O) high-alumina olivine tholeiite (HAOT);medium-K. calc-alkaline basalt, basaltic andesite, and andesite;and shoshonitic basaltic andesite (2?1% K₂O, 1750 ppm Sr at54% SiO₂). Tholeiites have MORB-like trace element abundancesexcept for elevated Sr, Ba, and Th and low high field strengthelements (HFSE), and they represent near-primary liquids. Theyare similar to HAOTs from the Cascades and adjacent Basin andRange, and to many primitive basalts from intraoceanic arcs.Calc-alkaline lavas show a well-developed arc signature of highlarge-ion lithophile elements (LILE) and low HFSE. Their Zrand Hf concentrations are at least partly decoupled from thoseof Nb and Ta; HREE are low relative to HAOT. Incompatible elementabundances and ratios vary widely among basaltic andesites.Some calc-alkaline lavas vented near Mount Mazama contain abundantgabbroic microxcnoliths, and are basaltic andesitic magmas contaminatedwith olivine gabbro. A calc-alkaline basalt and a few basaltic andesites have MgOand compatible trace element contents that suggest only minorfractionation. There appears to be a compositional continuumbetween primitive tholeiitic and calc-alkaline lavas. Compositionalvariation within suites of comagmatic primitive lavas, boththoleiitic and calc-alkaline, mainly results from differentdegrees of partial melting. Sources of calc-alkaline primarymagmas were enriched in LILE and LREE by a subduction componentand contained residual garnet, whereas sources of HAOTs hadlower LILE and LREE concentrations and contained residual clinopyroxene.High and variable LILE and LREE contents of calc-alkaline lavasreflect variations in fluid-transported subduction componentadded to the mantle wedge, degree of partial melting, and possiblyalso interaction with rocks or partial melts in the lower crust. Andesites were derived from calc-alkaline basaltic andesitesby fractionation of plagioclase+augite+magnetite+apatite ? orthopyroxeneor olivine, commonly accompanied by assimilation. Many andesitesare mixtures of andesitic or dacitic magma and a basaltic orbasaltic andesitic component, or are contaminated with gabbroicmaterial. Mingled basalt, andesite, and dacite of Williams Craterformed by multi-component, multi-stage mixing of basaltic andesiticmagma, gabbro, and dacitic magma. The wide range of compositionsvented from monogenetic volcanoes near Crater Lake is a resultof the thick crust coupled with mild tectonic extension superimposedon a subduction-related magmatic arc.     

中国东南部晚中生代-新生代玄武岩与壳幔作用

中国东南部的火山活动在早中生代时期仅有很小规模,晚中生代最早的、较大规模的岩浆活动始于中侏罗世早期,至早白垩世是火山岩浆活动的鼎盛期,在近100个Ma的时间内形成了大面积分布的晚中生代火山-侵入岩,而在新生代则以面积较小的玄武岩浆喷出活动为主,局限分布于沿海一带。以晚中生代湘南、赣南和闽西南的近EW向火山岩带和浙、闽沿海地区的近NNE向火山岩带,以及新生代近NNE向火山岩带为研究对象,对这些火山岩的地球化学特征对比研究,结合时空分布,讨论了它们的起源及其与壳幔相互作用的关系,以及它们形成的构造环境,其结果显示,EW向晚中生代火山岩带（180～170Ma）的西段玄武岩独立产出,且明显属碱性系列;而中段和东段玄武岩和流纹岩伴生,其中的玄武岩均为亚碱性系列的拉斑玄武岩。它们形成于板内拉张构造环境,是中国东南部特提斯构造域向太平洋构造域转换、晚中生代大规模岩浆作用的序幕。研究表明,该火山岩带自西向东表现出不同程度的壳幔相互作用,玄武岩在成岩过程中有少量陆壳组分加入。NNE向晚中生代火山岩带（130～90Ma）主要为流纹质岩石,安山岩和玄武岩很少。即使是双峰式火山岩也以酸性岩为主,玄武岩仅占全部火山岩体积的30％以下。其中的玄武岩主要属钙碱性系列,少数属拉斑系列。它们形成于火山弧构造环境,是中国东南部受太平洋构造域影响发生大规模火山岩浆作用的主旋律。其中玄武岩岩浆成分受到了较高程度的陆壳物质混染,同时代的中性火山岩是由底侵的玄武岩岩浆和陆壳物质来源的酸性岩浆发生岩浆混合作用而形成的,反映了强烈的壳幔相互作用。NNE向新生代火山岩带,分布在浙闽沿海,以碱性系列玄武岩为主,均含幔源包体,并受NNE向大陆边缘断裂构造的控制。它们形成于板内裂谷环境,是中国东南沿海由晚中生代火山弧构造环境转换为新生代板内裂谷环境的标志,起源于软流圈地幔,并有EMII岩石圈地幔的混合组分,但基本没有受到陆壳物质的混染。     

Mineralogy, Petrology, and Geochemistry of an Ultrapotassic Basaltic Suite, Central Sierra Nevada, California, U.S.A.

Ultrapotassic basaltic lavas erupted 3.4–3.6 m.y. ago(K/Ar) in the central Sierra Nevada and originated by partialmelting of a phlogopite-enriched, garnet-bearing upper mantlesource. Ultrapotassic basanites (K₂O: 5–9 per cent), whichare spatially related to contemporaneous potassic olivine basalts(K₂O: 3–5 per cent) and alkali olivine basalts (K₂O: 1–3per cent), contain the K₂O-bearing minerals phlogopite, sanidine,and leucite as well as olivine, diopside, apatite, magnetite,and pseudobrookite. The presence and modal abundance of theK₂O-bearing minerals closely reflects the east to west increasein K₂O throughout the basaltic suite. Many lines of evidence support the derivation of the ultrapotassicbasanites and the related basalts from an upper mantle source:TiO₂ in phlogopite phenocrysts and groundmass crystals, 2–3and 7–9 per cent respectively, support phlogopite phenocrystcrystallization at high pressure, whole rock Mg values (100Mg/Mg + 0.85 Fe) range from 66–78, phlogopite-rich pyroxeniticand periodotitic nodules are included in some flows, and geobarometriccalculations indicate depths of generation at 100–125km. Also, model calculations show that the major, rare earth,and trace elements, except for Ba, Rb, and Sr, can be accuratelygenerated by 1.0–2.5 per cent melting of a phiogopite-and garnet-bearing clinopyroxene-rich upper mantle source. Partialmelting occurred after a general upper mantle enrichment beneaththe Sierra Nevada, the phlogopite- and clinopyroxene-rich sourceof the ultrapotassic lavas being the extreme result of the enrichmentprocess. Clinopyroxene enrichment of the upper mantle probablyoccurred by introduction of a partial melting fraction intothe upper mantle source areas. Enrichment of the upper mantlein the alkali and alkali-earth elements was not accomplishedby a partial melt, but resulted from influx of a fluid phaserich in Ba, K, Rb, Sr, and, probably, H₂O The continuous rangein K₂O of the erupted lavas implies that the upper mantle enrichmentis a cumulative process. The inverse relationship in the SierraNevada between uplift and the K₂O content of the erupted basaltsimplies that a critical relationship may exist between upliftand upper mantle enrichment.     

青藏高原北部巴颜喀拉与东昆仑地区新近纪以来火山岩的地球化学特征及其成因

青藏高原北部巴颜喀拉构造区和东昆仑构造区,新生代火山岩形成于中新世、上新世和更新世,岩性为碱玄岩、玄武粗安岩、粗面安山岩（或安粗岩）、粗面岩、粗面英安岩、流纹斑岩和流纹岩。火山岩的分布与区域深大断裂的位置关系密切,与下伏地层之间均为角度不整合接触。火山岩为钙碱性、高钾钙碱性系列和钾玄岩系列。根据最新的火山岩年代资料和分布特征,研究区火山岩可划分为三个大的区域：①可可西里·鲸鱼湖地区,以可可西里湖北为中心向“四周”时代逐渐变新;②木孜塔格．银石山地区,以银石山西南为中心向“四周”时代逐渐变新;③黑石北湖地区。三个区域相比较,其火山岩的范围与规模从东向西变小,时代从东向西变新。微量元素的K、Rb、Ba、Th活动性元素及La、Ce轻稀土元素富集明显,高场强元素Nb、Ta、Ti亏损。流纹斑岩和流纹岩具强的负Eu异常,δEu的变化范围为0．04～0．39,其他岩石具有弱的负Eu异常,δEu值为0．61～0．95,经球粒陨石标准化,稀土分布曲线均为右倾负斜率轻稀土强烈富集型。从微量元素特征和稀土元素特征判断具有岛弧火山岩的成分特征。研究区从东向西火山岩从酸性到中基性演化,伴随这一规律各类岩石Na2O和K2O的平均值总体呈减小的趋势,TiO2、Al2O3、TFe、MnO、MgO、CaO、P2O5、δSr、Sr／Y、∑BEE、δEu、La／Yb平均值呈增加趋势,对于同一类火山岩,MgO随时代变新含量降低。部分火山岩的Sr、Nd同位素反映出富集Ⅱ型地幔（EMⅡ）的地球化学特征。包体特征显示,具有少量浅源包体,又具有深源包体。研究区火山岩形成于大陆内部,可能是在青藏高原自中新世以来的隆升过程中形成的,可能与青藏高原岩石圈减薄或大陆内部的俯冲过程有关。     

Constraints on the origin of alkaline and calc-alkaline magmas from the Tuxtla Volcanic Field,Veracruz, Mexico

 Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma include primitive basanites and alkali basalts, mildly alkaline Hy-normative mugearites and benmoreites, and calc-alkaline basalts and basaltic andesites. The primitive lavas are silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, variable La/Yb with constant Yb at 6 to 8 times chondritic, and low Sr and O and variable Pb and Nd isotopic ratios. The primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar, in the garnet stability field. Another group of alkali basalts and hawaiites has lower Ni and Cr concentrations and higher Fe/Mg ratios, and was derived from the primitive group by crystal fractionation at pressures of several kbar. Incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements (HFSE) relative to large ion lithophile elements, similar to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residual HFSE-bearing phase in the source, but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere. The silica-saturated mugearites and benmoreites, and the calc-alkaline basalts and basaltic andesites, were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations generally lower than in the silica-undersaturated lavas, and thus could not have been derived by crystal fractionation from the silica-undersaturated alkaline magmas. Magmas parental to the silica-saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas. Melting may have been promoted by an influx of fluid from the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotopic data suggest that three components are involved in the generation of TVF magmas: the mantle, a fluid from the subducted lithosphere, and continental crust. TVF alkaline lavas are similar to those erupted in the back-arc region of the MVB and Japan, and show characteristics similar to alkaline magmas erupted in the southern Andean volcanic arc. These low degree melts reach the surface along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passage to the surface. Received: 15 September 1994/Accepted: 14 February 1995     

Petrogenesis of Cenozoic Basaltic Rocks from Jiangsu Province,China:Evidence from Geochemical Constraints

Cenozoic（Miocene to Pleistocene） basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt.We present major,trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area.On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province,we suggest that these basalts may belong to the same magmatic system.The alkali basalts found in Jiangsu province have higherΣFeO,MgO,CaO,Na₂O, TiO₂ and P₂O₅ and incompatible elements,but lower Al₂O₃ and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine,pyroxene and minor plagioclase.In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/ΣFeO ratios.The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB.Partial loss of the mantle lithosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma.The ¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EMI component in the study area.According to Shaw’s equation,the basalts from Jiangsu province may be formed by l%-5%partial melting of a depleted asthenospheric mantle source.On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen（1982）,we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts.     

Chemostratigraphy of lava sequences from the Rio Itapicuru Greenstone Belt,Bahia State,Brazil

The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO₂ contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts.     

Petrogenesis of Silicic Rocks from the Krksfjrdur Central Volcano, NW Iceland

Calc-alkaline dacites are found in the Tertiary Kroksfjordurcentral volcano besides the more typical tholeiitic dacitesand rhyolites characteristic of Icelandic rift zones. It isclear that these calc-alkaline dacites are not subduction related,but their chemistry and petrography are definitely calc-alkaline.They are much lower in Fe and higher in Ca and Al than otherrocks from Iceland with comparable silica percentages. Theyare significantly depleted in the high field strength elements(HFSE, e.g., Nb, Zr, Y, and heavy rare earth elements), andsome of them contain hydrous phases or their relics. The anhydrousphase assemblage and relatively high Fe content of the tholeiiticsilicic rocks indicate generation at shallow depths (P_H2o <1kb). The high Ca and Al contents and the depletion of HFSE inthe calc-alkaline dacites indicate generation by partial meltingof amphibolite facies rocks at 5–6 km depth. The generationof the tholeiitic silicic rocks requires a shallow magma chamber,where they could be formed by fractionation of basaltic magma,or by partial melting of country rock heated by basaltic magma.The calc-alkaline dacites require a different volcanotectonicenvironment for their generation. The geothermal gradient requiredis comparable with that of regional geothermal gradients closeto the active rift zones. They were probably formed when thecentral volcano was drifting away from the rift axis and itsactivity was waning. Intrusion of basaltic magma, probably relatedto another volcanic center, mobilized the dacite magma.     

闽中地区马面山群东岩组变质岩形成的古构造环境研究

闽中地区马面山群东岩组地层主要为绿片岩为主的一套古火山沉积建造。其主要岩性类型包括各种成分的绿片岩、大理岩、石英片岩及变粒岩类。绿片岩显示海底火山喷发特征,变粒岩原岩为中酸性岩类。东岩组变质岩岩石化学研究表明,绿片岩的原岩应为玄武岩类。变粒岩类主要属于英安岩及流纹岩。这些特征反映东岩组具双峰式火山岩特征,形成于大陆内部张性环境。绿片岩稀土元素特征也显示和大陆拉张环境中的火山岩类稀土特征非常相似,属大陆拉斑玄武岩;微量元素分布显示出该组变质岩原岩类似于大洋岛和大陆裂谷的板内碱性玄武岩。因此闽中地区中元古代可能处于板内古裂谷环境。