Petrogenesis of Late Archaean Flood-Type Basic Lavas from the Klipriviersberg Group, Ventersdorp Supergroup, South Africa

The Klipriviersberg Group is a small continental flood-typetholeiitic suite forming the basal unit of the Ventersdorp Supergroup,an undeformed late-Archaean supracrustal sequence covering 200000km² in the SW part of the Kaapvaal Craton. From the base up,the Klipriviersberg Group consists of the Westonaria, Alberton,Orkney, Jeannette, Loraine, and Edenville formations, with amaximum combined thickness of 1–8 km. Samples were obtainedfrom several borehole cores in the Klerksdorp goldfield closeto the type area of the Klipriviersberg Group. This sample suiteincludes a detailed collection from a 1–36-km core intersection.These samples reveal that the Westonaria Formation is absentfrom the succession and that the Jeannette Formation is representedby a thin sequence of breccias. The remaining units are builtof lavas ranging from siliceous picrite basalts to tholeiiticandesites in composition. All lavas exhibit the effects of greenschistfacies metamorphism but igneous textures are well preservedand reveal that most lavas are aphyric to sparsely porphyritic.A well-developed geochemical stratigraphy exists in the lavasequence and is characterized by an upward trend from evolvedto primitive lavas. The Alberton (mg-number = 0.53–0.43;Zr= 108–137 ppm) and Orkney (mg-number = 0.51–O.42;Zr = 90–110ppm) lavas have a narrow compositional rangeand are more differentiated than the compositionally more variableLoraine/Edenville lavas (mg-number = 0.75 0.47; Zr = 34–97ppm). Unusual inter-element correlations in the Alberton andOrkney lavas suggest that magma mixing processes were importantin their petrogenesis. Compositional variation in the Loraine/Edenvillelavas is consistent with simple fractional crystallization dominatedby orthopyroxene. Collectively, all these lavas are light rareearth element (LREE)-enriched and have negative Nb-Ta, P, andTi anomalies on mid-ocean ridge basalt (MORB)- or primitivemantle-normalized abundance diagrams. In this they resemblemafic lavas from modern subduction-zone environments and manyArchaean and early Proterozoic siliceous high-magnesium basalts.The Klipriviersberg lavas also have initial Nd-isotopic compositionsthat cluster about the chondritic uniform reservoir (CHUR) valueregardless of degree of differentiation. It is unlikely thatcrustal contamination has influenced the composition of theKlipriviersberg lavas to any significant extent, and their geochemicalfeatures have probably been inherited from lithospheric mantlesources.     

Tourmaline in Proterozoic Massive Sulfide Deposits from Rajasthan, India

We have analyzed the chemical composition and boron isotope composition of tourmaline from tourmalinites, granite and a quartz-tourmaline vein from the Deri ore zone and from a pegmatitic band in the Rampura-Agucha ore body. These two Proterozoic massive sulfide deposits occur in the Aravalli-Delhi orogenic belt, Rajasthan, northwest India. Tourmaline from stratiform tourmalinites closely associated with the massive sulfides in the Deri deposit have preserved their original chemical compositions despite regional and thermal metamorphism in the area. These tourmalines have low Fe/(Fe + Mg) ratios (0.19–0.30; mean 0.26) that suggest formation close to the sediment-sea water interface. The δ¹¹B values (−15.5 and −16.4‰) are compatible with boron derived from leaching of argillaceous sediments and/or felsic volcanics underlying the original massive sulfide deposit during its formation. Boron isotope compositions measured in tourmaline from a post-ore granite and quartz-tourmaline vein in the Deri deposit indicate that boron in these tourmalines was derived from the tourmalinites produced during ore formation. The boron isotope systematics of a coarse brown tourmaline crystal from a pegmatitic band on the hanging wall contact of the Rampura-Agucha deposit indicate that 45 ± 25% of the boron within the original tourmaline was lost during upper amphibolite facies regional metamorphism. Received: 3 April 1996 / Accepted: 11 April 1996     

Petrology and Mineralogy of Eclogite and Garnet Amphibolite from Puerto Cabello, Venezuela

Occurrences of eclogite, eclogite-amphibolite, and garnet amphibolitefrom Puerto Cabello, Venezuela are described. Chemical analysesof thirteen rocks, eight garnets, six clinopyroxenes, and nineamphiboles are presented. Field evidence shows that the eclogitesand associated mafic rocks occur as sheets or lenses conformablewith pelitic and calcareous rocks and have been metamorphosedin situ. The associated country rocks have attained a metamorphicmineral assemblage typical of the epidote-amphibolite facies.The chemical composition of the eclogite and associated rocksis variable but is essentially that of tholeiitic basalt. Clino-pyroxenesfrom the eclogites are omphacites containing 34–51 moleper cent jadeite and the remainder largely diopside; coexistinggarnets contain approximately 50 mole per cent almandine, 24mole per cent pyrope, 25 mole per cent grossular+andradite,and 1 mole per cent spessartine. Determinations for the temperatureof metamorphism of these rocks give approximately 525±50°C. Determinations of total pressure of metamorphism, basedon the jadeite content of omphacite, on glaucophane II, kyanite,and calcite stability fields, and on stratigraphic reconstructionsgive a range of values between 5 and 10 kb with a most probablevalue near 7 kb. These data suggest a moderate geothermal gradientduring metamorphism of approximately 20 °C/km. An evaluationof the role of volatiles during metamorphism indicates thatthe original basalts were converted to eclogites in an environmentclosed to water and carbon dioxide. The field relationshipsat Puerto Cabello demonstrate the general instability of eclogite;the great majority of mafic rocks are amphibolites, garnet amphibolites,and eclogite-amphibolites. Eclogites, sensu stricto, are preservedonly in random ‘dry’ pockets within the stratigraphicsection containing the metabasalts. The transformation of basaltto eclogite to amphibolite is considered to have been a continuousparagenetic sequence during a single pseriod of metamorphism.     

Petrology and geochemistry of Camiguin Island, southern Philippines: insights to the source of adakites and other lavas in a complex arc setting

Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines. The island consists of four volcanic centers which have erupted basaltic to rhyolitic calcalkaline lavas during the last ∼400 ka. Major element, trace element and Sr, Nd and Pb isotopic data indicate that the volcanic centers have produced a single lava series from a common mantle source. Modeling results indicate that Camiguin lavas were produced by periodic injection of a parental magma into shallow magma chambers allowing assimilation and fractional crystallization (AFC) processes to take place. The chemical and isotopic composition of Camiguin lavas bears strong resemblance to the majority of lavas from the central Mindanao volcanic field confirming that Camiguin is an extension of the tectonically complex Central Mindanao Arc (CMA). The most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab. Some Camiguin high-silica lavas are similar to high-silica lavas from Mindanao, which have been identified as “adakites” derived from direct melting of a subducted basaltic crust. More detailed comparison of Camiguin and Mindanao adakites with silicic slab-derived melts and magnesian andesites from the western Aleutians, southernmost Chile and Batan Island in northern Philippines indicates that the Mindanao adakites are not pure slab melts. Rather, the CMA adakites are similar to Camiguin high-silica lavas which are products of an AFC process and have negligible connection to melting of subducted basaltic crust. Received: 27 February 1998 / Accepted: 27 August 1998     

Zeolite and prehnite-pumpellyite facies in south central Puerto Rico

The intermediate lavas and pyroclastic rocks of south central Puerto Rico have been subjected to about four kilometers of burial. Despite shallow burial the mineralogy of these rocks has systematically readjusted. The degree of metamorphism is proportional to permeability; secondary phases appear in greater quantities in porous pyroclastics. In lavas, plagioclase and olivine phenoerysts contain a progressive sequence of alteration phases reflecting temperature and pressure conditions during alteration. The generalized sequence of appearance of secondary phases from low to high rank is as follows: 1. analcime, celadonite, chlorite, and sericite; 2. laumontite and albite; 3. prehnite; 4. pumpellyite; 5. epidote; 6. actinolite. Assemblages containing analcime, heulandite, celadonite, and laumontite belong to the zeolite facies. Remaining assemblages belong to the prehnite-pumpellyite facies.During alteration two major thresholds were crossed. First, calcium-aluminum silicates formed from materials released by decomposition of calcium feldspar. Second, clinopyroxene was decomposed in a process that added considerable mafic material to the reacting system, and made bulk rock compositions approximately equivalent to the composition of the reacting system. Comparison with other regions of similar metamorphic rank and composition indicates that zonation of such sequences on the basis of individual mineral occurrences, especially epidote, is justified only for local regions where the behavior of volatiles was uniform.Based on part of a dissertation (Jolly, 1969) presented to the faculty of the Department of Geology, State University of New York at Binghamton.     

Vector analysis of chemical variation in the lavas of Parícutin volcano,Mexico

Compositional variations in the lavas of Parícutin volcano, Mexico, have been examined by an extended method of Q-mode factor analysis. Each sample composition is treated as a vector projected from an original eight-dimensional space into a vector system of three dimensions. The compositions represented by the vectors after projection are closely similar to the original compositions except for Na₂Oand Fe₂O₃.The vectors in the three-dimensional system cluster about three different planes that represent three stages of compositional change in the Parícutin lavas. Because chemical data on the compositions of the minerals in the lavas are presently lacking, interpretations of the mineral phases that may have been involved in fractional crystallization are based on CIPW norm calculations. Changes during the first stage are attributed largely to the fractional crystallization of plagioclase and olivine. Changes during the second stage can be explained by the separation of plagioclase and pyroxene. Changes during the final stage may have resulted mostly from the assimilation of a granitic material, as previously proposed by R. E. Wilcox.     

Composition of geothermal minerals from saline and dilute fluids — Krafla and Reykjanes, Iceland

In high temperature geothermal areas (T.200°C) in Iceland the alteration minerals formed show a regular zoning with increasing temperature i.e. smectite, chlorite, epidote and actinolite. To test the effect of varying composition of the hydrothermal circulation fluid drillhole chips from two contrasting areas were analyzed in detail. The Reykjanes area is fed with sea water while Krafla is fed with meteoric water and has a Cl⁻ concentration of 28 ppm. Both areas have tholeiitic basalts, lavas and hyaloclastites as original reservoir rocks.

Although both systems have the same sequence of alteration minerals the actual composition of the minerals differs between systems reflecting the different chemical composition of the circulating waters. In Reykjanes, with its brine fluid, the composition of the original rock appears not to affect the composition of the alteration minerals while the reverse appears to be the case in Krafla.     

Long length scales of element transport during reaction texture development in orthoamphibole-cordierite gneiss: Thor-Odin dome,British Columbia,Canada

First-order factors controlling the textural and chemical evolution of metamorphic rocks are bulk composition and pressure–temperature–time (P–T–t) path. Although it is common to assume that major element bulk composition does not change during regional metamorphism, rocks with reaction textures such as corona structures record evidence for major changes in effective bulk composition (EBC) and therefore provide significant insight into the scale, pathways, and mechanisms of element transport during metamorphism. Quantifying changes in EBC is essential for petrologic applications such as calculation of phase diagrams (pseudosections). The progressive growth of complex corona structures on garnet and Al₂SiO₅ porphyroblasts in orthoamphibole-cordierite gneiss Thor-Odin dome (British Columbia, Canada) reduced the EBC volume of the rock during metamorphism and therefore had a dramatic effect on the evolution of the stable mineral assemblage. These rocks contain a chemical and textural record of metamorphic reactions and preserve 3D networks (reaction pathways) connecting corona structures. These coronal networks record long (>cm) length scales of localized element transport during metamorphism. P–T, T–X, and P–X pseudosections are used to investigate the control of effective bulk composition on phase assemblage evolution. Despite textural complexity and evidence for disequilibrium, mineral assemblages and compositions were successfully modeled and peak metamorphic conditions estimated at 750°C and 9 kbar. These results illustrate how textural and chemical changes during metamorphism can be evaluated using an integrated petrographic and pseudosection approach, highlight the importance of effective bulk composition choice for application of phase equilibria methods in metamorphic rocks, and show how corona structures can be used to understand the scale of compositional change and element transport during metamorphism.     

The Composition and Origin of the Striped Amphibolites of Connemara, Ireland

Major- and trace-element analyses, accompanied by modal analysesand mineralogical data, are given for thirty-three striped amphibolitesfrom Connemara. It is concluded that prior to metamorphism theserocks were intrusions, lavas or tuffs of sodic basalt composition.     

Generation of Deccan Trap magmas

Deccan Trap magmas may have erupted through multiple centers, the most prominent of which may have been a shield volcano-like structure in the Western Ghats area. The lavas are predominantly tholeiitic; alkalic mafic lavas and carbonatites are rare. Radioisotope dating, magnetic chronology, and age constraints from paleontology indicate that although the eruption started some 68 Ma, the bulk of lavas erupted at around 65–66 Ma. Paleomagnetic constraints indicate an uncertainty of ± 500,000 years for peak volcanic activity at 65 m.y. in the type section of the Western Ghats. Maximum magma residence times were calculated in this study based on growth rates of “giant plagioclase” crystals in lavas that marked the end phase of volcanic activity of different magma chambers. These calculations suggest that the > 1.7 km thick Western Ghats section might have erupted within a much shorter time interval of ∼ 55,000 years, implying phenomenal eruption rates that are orders of magnitude larger than any present-day eruption rate from any tectonic environment. Other significant observations/conclusions are as follows: (1) Deccan lavas can be grouped into stratigraphic subdivisions based on their geochemistry; (2) While some formations are relatively uncontaminated others are strongly contaminated by the continental crust; (3) Deccan magmas were produced by 15–30% melting of a Fe-rich lherzolitic source at ∼ 3–2 GPa; (4) Parent magmas of the relatively uncontaminated Ambenali formation had a primitive composition with 16%MgO, 47%SiO₂; (5) Deccan magmas were generated much deeper and by significantly more melting than other continental flood basalt provinces; (6) The erupted Deccan tholeiitic lavas underwent fractionation and magma mixing at ∼ 0.2 GPa. The composition and origin of the crust and crust/mantle boundary beneath the Deccan are discussed with respect to the influence of Deccan magmatic episode.     

The Composition and Origin of the Striped Amphibolites of Connemara, Ireland

Major- and trace-element analyses, accompanied by modal analysesand mineralogical data, are given for thirty-three striped amphibolitesfrom Connemara. It is concluded that prior to metamorphism theserocks were intrusions, lavas or tuffs of sodic basalt composition.     

The Mafic and Ultramafic Lavas of the Belingwe Greenstone Belt, Rhodesia

The Belingwe Greenstone Belt (2.8 x 10⁹ yrs old) contains a7 km succession of mafic and ultramafic lavas and high-levelintrusions which overlie a thin sedimentary formation, itselfunconformable on a granitic basement. The lavas range in compositionfrom andesites (4 per cent MgO) to peridotitic komatiites (32per cent MgO). The mineralogy and textures of the most magnesianlavas demonstrate that they were extruded in a completely liquidstate. If the source mantle had an MgO content around 40 percent, then partial melts in the range 35 per cent to 55 percent would be required to produce the most magnesian liquidsobserved. Chemical constraints on the petrogenesis of the ultramafic lavasallow estimates of source mantle composition. In particular,if the source had an MgO content around 40 per cent, then theoverall source composition would be similar to that of garnetIherzolite nodules in kimberlites. The calculated REE contentsof the source are close to chondritic. If all the ultramaficlavas were derived from the same source then the variation inliquid composition may have been controlled by orthopyroxeneas well as olivine during partial melting at depth. The evolutionof the less magnesian komatiites, basalts, and andesites canbe explained by lower degrees of partial melting of a commonsource, and by high-level fractionation of parent liquids similarto those extruded as ultramafic lavas. Physical constraints on the origin of the lavas imply derivationfrom a depth of 150 km or more, at temperatures of 1600–2000°C.     

Magma Genesis and Mantle Dynamics at the Harrat Ash Shamah Volcanic Field (Southern Syria)

A geochemical and petrological study of Miocene to recent alkalibasalts, basanites, hawaiites, mugearites, trachytes, and phonoliteserupted within the Harrat Ash Shamah volcanic field was performedto reconstruct the magmatic evolution of southern Syria. Themajor element composition of the investigated lavas is mainlycontrolled by fractional crystallization of olivine, clinopyroxene,± Fe–Ti oxides and ± apatite; feldspar fractionationis restricted to the most evolved lavas. Na₂O and SiO₂ variationswithin uncontaminated, primitive lavas as well as variably fractionatedheavy rare earth element ratios suggest a formation by variabledegrees of partial melting of different garnet peridotite sourcestriggered, probably, by changes in mantle temperature. The isotopicrange as well as the variable trace element enrichment observedin the lavas imply derivation from both a volatile- and incompatibleelement-enriched asthenosphere and from a plume component. Inaddition, some lavas have been affected by crustal contamination.This effect is most prominent in evolved lavas older than 3·5Ma, which assimilated 30–40% of crustal material. In general,the periodicity of volcanism in conjunction with temporal changesin lava composition and melting regime suggest that the Syrianvolcanism was triggered by a pulsing mantle plume located underneathnorthwestern Arabia. KEY WORDS: ⁴⁰Ar/³⁹Ar ages; intraplate volcanism; mantle plume; partial melting; Syria     

Petrogenesis of Tholeiitic Lavas from the Submarine Hana Ridge, Haleakala Volcano, Hawaii

Hana Ridge, the longest submarine rift zone in the Hawaiianisland chain, extending from Maui 140 km to the ESE, has a complexmorphology compared with other Hawaiian rift zones. A totalof 108 rock specimens have been collected from the submarineHana Ridge by six submersible dives. All of the rocks (76 bulkrocks analyzed) are tholeiitic basalts or picrites. Their majorelement compositions, together with distinctively low Zr/Nb,Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast,the lavas forming the subaerial Honomanu shield are intermediatein composition between those of Kilauea and Mauna Loa. The compositionalcharacteristics of the lavas imply that clinopyroxene and garnetwere important residual phases during partial melting. The compositionsof olivine and glass (formerly melt) inclusions imply that regardlessof textural type (euhedral, subhedral–undeformed, deformed)olivine crystallized from host magmas. Using the most forsteriticolivine (Fo_90·6) and partition coefficients     

The Metamorphism of Fe-rich Pelites from Connemara, Ireland

Connemara pelites show progressive metamorphism from stauroliteto upper sillimanite zones and possess low Mg/(Fe + Mg) values,typically 0.30 to 0.35 from about 100 analyses. As a consequenceof their composition, many sillimanite zone pelites lack bothmuscovite and K-feldspar. Staurolite, garnet, biotite, muscovite,feldspars and iron ores have been microprobe analysed in 48samples. Assemblages, textures and mineral compositions indicatethat metamorphism followed a sequence of continuous and discontinuousreactions with systematic variations in mineral Mg/(Mg + Fe)as predicted by theory. Contrary to some common assumptions,most reaction takes place along divariant equilibria; univariantreactions are seldom reached because reactants such as chloriteor muscovite are first consumed along divariant curves. Pelitepetrogenetic grids showing univariant curves can only indicatelimits to natural assemblages; they typically do not show whichreactions have actually taken place. Physical conditions of metamorphism have been calculated bya variety of means; temperatures range from 550° for thestaurolite zone to 650° for the upper silimanite zone, withthe first appearance of sillimanite near 580°. An earlykyanite-staurolite metamorphism at pressures above about 5 kbwas followed by a steepening of the thermal gradient leadingto regional cordierite and andalusite. This was probably accompaniedby uplift with pressures of around 4 kb for roeks near the sillimanite-inisograd.     

Geochemistry and tectonic setting of early Pan-African metamorphosed volcano–sedimentary sequence in southern Solaf zone, SW Sinai, Egypt

The south Solaf zone in SW Sinai comprises a metasedimentary sequence of metagraywackes intercalated with minor metavolcanic sheets, metasiltstones, meta-arenites, and calc–silicates. The metavolcanics (basalt–andesite) show high- to medium-K calc–alkaline nature. They exhibit distinctive Nb-Ta negative anomalies relative to enriched LILE, being highly similar to active continental margin lavas, but they also have the characteristics of rift-related magmatism. Magmas of similar composition are interpreted to be formed in an extensional environment and their source regions are zones of enriched subcontinental lithosphere. The metasediments are poor to moderate sorting, intercalated to the north with minor impure calcareous layers. Geochemical investigation shows that they are immature to semi-mature sediments derived from a source of mafic to felsic composition. These metasediments are chemically similar to the active continental margins and are comparable to the Feiran gneisses and metagraywackes that were deposited before 800 Ma in an extensional environment. The investigated rocks suffered LP-HT amphibolite facies metamorphism. The P-T estimates using various thermobarometric calibrations gave temperatures of 554–610 °C and pressures of 2.2–4.0 kbar.     

Geochemistry of the Active Azufre--Planchon--Peteroa Volcanic Complex, Chile (3515'S): Evidence for Multiple Sources and Processes in a Cordilleran Arc Magmatic System

Along strike of the Quaternary magmatic arc in the SouthernVolcanic Zone of the Andes, there is a south to north increasein crustal thickness, and the lavas define systematic geochemicaltrends which have been attributed to variations in the proportionsand compositions of mantle-and crustal-derived components. Realisticinterpretations of these regional geochemical trends requiresan understanding of the sources and processes that control lavacompositions at individual volcanoes. Because it is in an importantgeophysical and geochemical transition zone, we studied theAzufre—Planchon—Peteroa volcanic complex, a nestedgroup of three volcanoes <055 m.y. in age located at 3515'Sin the Southern Volcanic Zone of the Andes. North of this complexat 33–35S the continental crust is thick, basalts areabsent, and there is abundant evidence for crustal componentsin the evolved lavas, but south of 37S, where the crust isrelatively thin, basaltic lavas are abundant and the contributionof continental crust to the lavas is less obvious. In additionto its location, this volcanic complex is important becausethere is a diversity of lava compositions, and it is the northernmostexposure of recent basaltic volcanism on the volcanic front.Therefore, the lavas of this complex can be used to identifythe relative roles of mantle, lower-crustal and upper-crustalsources and processes at a single location. Volcan Azufre is the oldest and largest volcano of the complex;it is a multi-cycle, bimodal, basaltic andesite–dacitestratovolcano. Volcan Planchon is the northernmost basalt-bearingvolcano along the volcanic front of the Southern Andes, andVolcan Peteroa, the youngest volcano of the complex, has eruptedmixed magmas of andesitic and dacitic composition. Most basalticandesite lavas at Azufre and Planchon are related by a plagioclase-poor,anhydrous mineral fractionating assemblage. High-alumina basaltis produced from a tholeiitic parent by an 4–8 kbar fractionatingassemblage. During this moderatepressure crystallization, themagmas also incorporated a crustal component with high La/Yband high abundances of Rb, Cs and Th. Based on the chemicalcharacteristics of the added component and the inferred depthof crystallization, the crustal source may have been garnetgranulite derived from solidified arc magmas in the lower tomiddle continental crust. At Planchon, the role of crustal assimilationhas increased with decreasing eruption age probably becausecrustal temperatures have increased during continued volcanism.Azufre dacite lavas formed at low pressures by fractionationof a plagioclase-rich assemblage. These dacite lavas containan upper-crustal component, probably derived in part from limestone,with high values of ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O. Thus two depths (upperand lower crust) of crystallization and associated crustal assimilationare evident in Planchon–Azufre lavas. Peteroa, the focusof recent volcanism, consists of calc-alkaline andesite anddacite eruptive products whose textures and compositions indicatean important role for magma mixing. Therefore, the volcanismevolved from a tholeiitic system of basalt and subordinate dacite(Planchon–Azufre) to a calc-alkaline system with abundantmixed lavas of intermediate composition (Peteroa). In additionto crustal thickness, two important parameters which controlledthe diversity of lava composition in this complex are magmasupply rate from the mantle and crustal temperature. Both parametersvaried with time, and they must be considered in broader interpretationsof along-strike geochemical trends. KEY WORDS: arc magmas; Andes; Peteroa; Planchan; geochemistry ^*Corresponding author. Present address: ENTRIX, Inc., 4II North Central Avenue, Glendale, CA 91203, USA     

滇西南涧地区无量山岩群变质英安岩锆石U-Pb年龄及其构造意义

滇西南涧—云县一带广泛出露的无量山岩群是一套与三江特提斯造山带密切相关的中低变质沉积岩夹火山岩系,其形成时代、沉积充填序列及大地构造属性一直存在争议。出露于南涧县公郎乡一带的无量山岩群发育有厚数厘米的变质英安岩夹层。对变质英安岩进行LA-ICP-MS锆石U-Pb定年,分别获得428.7±8.7Ma的岩浆结晶年龄和338～387Ma、250.5±8.1Ma、146.0±6.0Ma的变质年龄。认为无量山岩群原始沉积岩形成于志留纪,在古特提斯洋俯冲过程中,经历古生代造弧而发生区域变质作用,在白垩纪受热事件影响而发生热变质作用;同时还获得2034±22Ma、1140±16Ma、731±15Ma及469～522Ma的单颗粒锆石年龄,这些年代峰值与整个扬子陆块西缘获得的前寒武纪、古生代构造热事件年龄及分布特征基本可以对比;记录了Columbia、Rodinia和Gondwana超大陆的形成与裂解过程。     

变质作用中原岩建造的地位及意义

变质作用是地壳历史发展中的一种重要的地质作用,并随地壳的演化而表现为不同的变质作用类型。过去称为区域变质作用可根据其所处的大地构造环境而分为不同的变质作用类型,而大地构造环境的一个重要方面则是它的原岩建造的特性。变质作用开始的大地构造环境一般可认为是具有活动特点的下沉带,表现为巨厚的