Petrology and petrogenesis of a tertiary bimodal dolerite-peralkaline/subalkaline trachyte/rhyolite dyke association from Lundy,Bristol Channel,UK

The island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite). The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.     

Petrochemistry of the claret creek ring complex,northeast Queensland

The Claret Creek Ring Complex is one of several calc‐alkaline ring complexes in a Carboniferous epizonal batholith emplaced into continental crust at the junction of the Precambrian Georgetown Inlier and the adjacent Palaeozoic Tasman Geo‐syncline, northeast Queensland. Rhyolite ash‐flow sheets plus rhyolite and dacite ring dykes are intruded by two comagmatic central stocks of microgranite and grano‐diorite‐tonalite. The complex may be chemically distinguished from the surrounding, contemporaneous batholith by its low K/Na, Rb/Sr and Th/K ratios. The origin and variation of its magmas is explained by invoking progressive partial melting of low K/Na basaltic andesites. Close relatives to the magma source‐rock are preserved as microdiorite xenoliths, which have contaminated their host granodiorite‐tonalite stock.     

SHRIMP geochronology for the 1450 Ma Lakhna dyke swarm: Its implication for the presence of Eoarchaean crust in the Bastar Craton and 1450–517 Ma depositional age for Purana basin (Khariar), Eastern Indian Peninsula

Zircon U–Pb ages of the Mesoproterozoic dyke swarms (Lakhna dyke swarm) at the interface between the Eastern Ghats Mobile Belt and Bastar Craton of the Indian Peninsula are reported here to decipher the tectonic evolution of the region. The dyke swarm, which is dominantly N–S in orientation, has intruded the Bastar Craton at ca. 1450 Ma. The dykes vary in composition from dolerite to trachyte and rhyolite and have been emplaced in a continental anorogenic setting. The above age puts a lower time constraint on the sedimentary sequences of the Purana basin (Khariar basin) that have been deposited unconformably over the Bastar Craton. The shale member of the Khariar basin shows evidence of synsedimentary shearing suggesting that the sedimentation probably continued up to 517 Ma, the age of shearing and overthrusting of the granulite nappes of the Eastern Ghats Mobile Belt on the Craton. Further, the compression accompanying thrusting of the nappes, uplifted the Purana basins during inversion.     

Evolution of the middle Proterozoic Chandabooka Caldera,Gawler range acid volcano‐plutonic province,South Australia

Volcanism associated with the middle Proterozoic Gawler Range acid volcano‐plutonic province was initiated in the Kokatha area by the construction on Archaean basement of a large stratovolcano composed mainly of tholeiitic basalt and potassic basaltic‐andesite erupted possibly from a mantle‐derived ultramafic diapir.

Crustal melting above the diapir generated acid magma, rich in silica and potassium, which rose by major block‐stoping to form a subvolcanic magma chamber. Leakage from this chamber during the premonitory caldera phase gave rise to small explosive and effusive eruptions around an incipient ring‐fracture zone. In the caldera phase, the eruption of voluminous rhyodacite to dacite ignimbrite from the subvolcanic magma chamber resulted in collapse of the roof partway through the eruption to form the Chandabooka caldera, 15 x 10 km across: the ignimbrite comprises a thick compound cooling unit, the Chandabooka Dacite, of which both the caldera and outflow facies are preserved. Resurgent doming and subsequent uplift of the caldera block by 1 km followed in the post‐caldera phase, accompanied by minor acidic volcanism. Flat‐roofed stocks of the primitive S‐type Hiltaba Granite and a major dyke swarm intruded the volcanic pile to complete the volcano‐plutonic episode.     

The form of the intrusive complex at mount dromedary,New South Wales

In the past the Mount Dromedary igneous complex has been regarded as a differentiated laccolith, with the more felsic banatite at the summit of the mountain, monzonite on the lower slopes and pyroxenite at sea‐level.

Evidence is put forward to show that this is not the case and that the form is that of a stock or ring‐dyke with very steep contacts.

The monzonite has been emplaced by forceful injection and the banatite by permissive emplacement along a vertical, cylindrical fracture in the monzonite. The pyroxenite may form a separate dyke or stock.     

Geochemical and geochronological study of the Sanshui basin bimodal volcanic rock suite,China: Implications for basin dynamics in southeastern China

Sanshui basin is one of the typical Mesozoic–Cenozoic intra-continental rift basins with voluminous Cenozoic volcanic rocks in southeastern China. Thirteen cycles of volcanic eruptions and two dominant types of volcanic rocks, basalt and trachyte–rhyolite, have been identified within the basin. Both basalt and trachyte–rhyolite members of this bimodal suit have high values of εNd (+2.3 to +6.2) and different Sr isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr ratios are 0.70461–0.70625 and 0.70688–0.71266 for basalts and trachyte–rhyolite, respectively), reflecting distinct magma evolution processes or different magma sources. The results presented in this study indicate that both of the trachyte–rhyolite and basaltic magmas were derived from similar independent primitive mantle, but experienced different evolution processes. The trachyte-rhyolitic magma experienced significant clinopyroxene and plagioclase fractionational crystallization from deeper magma chamber with significant crustal contamination, while the basaltic magmas experienced significant olivine and clinopyroxene fractionational crystallization in shallower magma chamber with minor crustal contamination. New zircon U–Pb dating confirms an initial volcanic eruption at 60 Ma and the last activity at 43 Ma. Geologic, geochemical, and geochronological data suggest that the inception of the Sanshui basin was resulted from upwelling of a mantle plume. The Sanshui basin widened due to subsequent east–west extension and the subsequent volcanism constantly occurred in the center of the basin. Evidence also supports a temporal and spatial association with other rift basins in southeastern China. The upwelling mantle plume became more active during late Cenozoic time and most likely triggered opening of other basins, including the young South China Sea basin.     

沽源火山盆地燕山期次流纹斑岩地质地球化学特征

燕山期次流纹斑岩主要分布在沽源火山盆地盖层的上侏罗统张家口组火山岩中,活动时间以燕山晚期为主.次流纹斑岩体在空间分布上受断裂构造和燕山晚期火山构造控制,与铀钼矿化关系密切,矿化主要产在次流纹斑岩体接触带或其顶部的隐爆角砾岩中.次流纹斑岩与围岩呈侵入接触关系,呈岩株、岩墙、岩脉等形态产出.岩石地球化学研究表明:次流纹斑岩...     

Age and origin of early Proterozoic dolerite dykes in South-West Greenland

Samples of three dolerite dykes from South-West Greenland give a pooled weighted mean Rb-Sr whole-rock isochron age of 2,130±65 Ma. Initial ⁸⁷Sr/⁸⁶Sr ratios are 0.70155±0.00018 for one of the dykes (Naujat) and 0.70277±0.00012 for the other two (Torssut and Akinaq). A fourth dyke (Qaqarssuaq) did not yield an isochron. Torssut samples have significantly lower initial ²⁰⁷Pb/²⁰⁴Pb ratios than samples from the Naujat dyke, indicating that the Torssut dyke has been affected by crustal contamination. Samples of the dykes vary widely in chemical composition because of an uneven distribution of different primocryst phases throughout the dykes, and because of variable degrees of fractionation of the magma. For two of the dykes some constraints on the composition of the parent magma have been inferred from trace element data. Higher concentrations of K, Rb, Ba, Sr, Ce and perhaps Nb (at comparable levels of magmatic fractionation) in the dykes with higher Sr_i are probably related to crustal contamination of their parent magmas. Two of the dykes probably intruded from zoned magma chambers. The marginal zones of these dykes crystallised from a strongly fractionated magma, whereas the centres of the dykes consist of more primitive material with large proportions of olivine and plagioclase primocrysts. The central part of Torssut is less strongly contaminated with crustal Pb than the border zones. A more detailed analysis of the Pb-isotopic composition in the Torssut dyke shows that contaminant lead must have been derived from rocks in the deeper crust considerably older than the late Archaean gneisses which occur at the surface.     

Emplacement and deformation of the Wyangala Batholith,New South Wales

The Wyangala Batholith, in the Lachlan Fold Belt of New South Wales, is pre‐tectonic with respect to the deformation that caused the foliation in the granite, and was emplaced during a major thermal event, perhaps associated with dextral shearing, during the Late Silurian to Early Devonian Bowning Orogeny. This followed the first episode of folding in the enclosing Ordovician country rocks. Intrusion was facilitated by upward displacement of fault blocks, with local stoping. Weak magmatic flow fabrics are present. After crystallization of the granite, a swarm of mafic dykes intruded both the granite and country rock, possibly being derived from the same tectonic regime responsible for emplacement of the Wyangala Batholith. A contact aureole surrounding the granite contains cordierite‐biotite and cordierite‐andalusite assemblages. Slaty cleavage produced in the first deformation was largely obliterated by recrystallization in the contact aureole.

Postdating granite emplacement and basic dyke intrusion, a second regional deformation was accompanied by regional metamorphism ranging from lower greenschist to albite‐epidote‐amphibolite facies, and produced tectonic foliations, termed S and C, in the granite, and a foliation, S₂, in the country rocks. Contact metamorphic rocks underwent retrogressive regional metamorphism at this time. S formed under east‐west shortening and vertical extension, concurrently with S₂. C surfaces probably formed concurrently with S and indicate reverse fault motion on west‐dipping ductile shear surfaces. The second deformation may be related to Devonian or Early Carboniferous movement on the Copperhannia Thrust east of the Wyangala Batholith.     

Petrology and geochemistry of the Banks Peninsula volcanoes,South Island,New Zealand

Within the volcanic sequence of the twin volcanoes of Lyttelton and Akaroa, Banks Peninsula, New Zealand a number of different magma series have been distinguished.An early series of hawaiites (McQueens Valley Formation) was erupted about 32 m.y. ago and is of transitional or mildly tholeiitic chemistry. Stratigraphically above the McQueens Valley Formation, but unconformably overlain by the main volcanic dome sequence, is a unit of rhyolite (Gebbies Pass Rhyolites) which is not directly related to the earlier or later basaltic volcanism. The rhyolite was probably formed during intracrustal melting which was related to the rise of basaltic magma into the crust.Between 12 and 9.7 m.y. a large volcanic dome, composed mainly of hawaiite, was built at Lyttelton. Dykes, which intrude the Lyttelton volcanic sequence, range in composition from basalt to trachyte. Late, mildly alkalic, basaltic flank flows (7.5–5.8 m.y.) occur in several areas and they, and the differentiated rocks of the dyke swarm can be related by a crystal fractionation model which has been quantitatively tested.Following construction of the Lyttelton dome a second larger dome was built at Akaroa between 9 and 7.5 m.y. The rocks of the Akaroa Volcano are principally hawaiites but rocks ranging in composition through to trachyte also occur. The differentiated rocks of the Akaroa volcano have derived from the basaltic rocks by a crystal fractionation controlled process, operating during ascent through the crust.None of the Banks Peninsula basalts appear to have derived from primitive (pyrolitic) mantle material, but progressive changes in the chemistry of the basalts with time implies that the mantle source regions were evolving geochemically as partial melting proceeded. Later lavas tend to be more alkalic and to have lower MgO/FeO ratios than earlier lavas. The volcanic rocks of the Banks Peninsula volcanoes were derived by fractional removal of olivine, plagioclase, clinopyroxene, magnetite and apatite from ascending basaltic magma batches. Variations between the suites reflect differences between the parental magma batches.     

Chemical and thermal zonation in a mildly alkaline magma system Infiernito Caldera,Trans-Pecos Texas

Postcollapse lavas of the Infiernito caldera grade stratigraphically upward from nearly aphyric, high-silica rhyolite (76% SiO₂) to highly prophyritic trachyte (62% SiO₂). Plagioclase, clinopyroxene, orthopyroxene, magnetite, ilmenite, and apatite occur as phenocrysts throughout the sequence. Sanidine, biotite, and zircon are present in rocks with more than about 67% SiO₂. Major and trace elements show continuous variations from 62 to 76% SiO₂. Modeling supports fractional crystallization of the observed phenocrysts as the dominant process in generating the chemical variation.Temperatures calculated from coexisting feldspars, pyroxenes, and Fe-Ti oxides agree and indicate crystallization from slightly more than 1100° C in the most mafic trachyte to 800° C in high-silica rhyolite. The compositional zonation probably arose through crystallization against the chilled margin of the magma chamber and consequent rise of more evolved and therefore less dense liquid.Mineral compositions vary regularly with rock composition, but also suggest minor mixing and assimilation of wall rock or fluids derived from wall rock. Mixing between liquids of slightly different compositions is indicated by different compositions of individual pyroxene phenocrysts in single samples. Liquid-solid mixing is indicated by mineral compositions of glomerocrysts and some phenocrysts that apparently crystallized in generally more evolved liquids at lower temperature and higher oxygen fugacity than represented by the rocks in which they now reside. Glomerocrysts probably crystallized against the chilled margin of the magma chamber and were torn from the wall as the liquid rose during progressive stages of eruption. Assimilation is indicated by rise of oxygen fugacity relative to a buffer from more mafic to more silicic rocks.Calculation of density and viscosity from the compositional and mineralogical data indicates that the magma chamber was stably stratified; lower temperature but more evolved, thus less dense, rhyolite overlay higher temperature, less evolved, and therefore more dense, progressively more mafic liquids. The continuity in rock and mineral compositions and calculated temperature, viscosity, and density indicate that compositional gradation in the magma chamber was smoothly continuous; any compositional gaps must have been no greater than about 2% SiO₂.     

Mixing of rhyolite,trachyte and basalt magma erupted from a vertically and laterally zoned reservoir,composite flow P1, Gran Canaria

The 14.1 Ma composite welded ignimbrite P1 (45 km³ DRE) on Gran Canaria is compositionally zoned from a felsic lower part to a basaltic top. It is composed of four component magmas mixed in vertically varying proportions: (1) Na-rhyolite (10 km³) zoned from crystal-poor to highly phyric; (2) a continuously zoned, evolved trachyte to sodic trachyandesite magma group (6 km³); (3) a minor fraction of Na-poor trachyandesite (<1 km³); and (4) nearly aphyric basalt (26 km³) zoned from 4.3 to 5.2 wt% MgO. We distinguish three sites and phases of mixing: (a) Mutual mineral inclusions show that mixing between trachytic and rhyolitic magmas occurred during early stages of their intratelluric crystallization, providing evidence for long-term residence in a common reservoir prior to eruption. This first phase of mixing was retarded by increasing viscosity of the rhyolite magma upon massive anorthoclase precipitation and accumulation. (b) All component magmas probably erupted through a ring-fissure from a common upper-crustal reservoir into which the basalt intruded during eruption. The second phase of mixing occurred during simultaneous withdrawal of magmas from the chamber and ascent through the conduit. The overall withdrawal and mixing pattern evolved in response to pre-eruptive chamber zonation and density and viscosity relationships among the magmas. Minor sectorial variations around the caldera reflect both varying configurations at the conduit entrance and unsteady discharge. (c) During each eruptive pulse, fragmentation and particulate transport in the vent and as pyroclastic flows caused additional mixing by reducing the length scale of heterogeneities. Based on considerations of magma density changes during crystallization, magma temperature constraints, and the pattern of withdrawal during eruption, we propose that eruption tapped the P1 magma chamber during a transient state of concentric zonation, which had resulted from destruction of a formerly layered zonation in order to maintain gravitational equilibrium. Our model of magma chamber zonation at the time of eruption envisages a basal high-density Na-poor trachyandesite layer that was overlain by a central mass of highly phyric rhyolite magma mantled by a sheath of vertically zoned trachyte-trachyandesite magma along the chamber walls. A conventional model of vertically stacked horizontal layers cannot account for the deduced density relationships nor for the withdrawal pattern.     

The zoned Bruinbun granitoid pluton and its aureole

The Bruinbun pluton is a small, massive, I‐type granitoid intruded into the meta‐sedimentary Hill End Trough, in eastern N.S.W. It is a multiple pluton representing two discrete magma pulses, the granodiorite core intruding the adamellite margin. A weak alignment of orthoclase megacrysts in the granitoids is best developed adjacent to both the internal and external intrusive contacts, and is considered by the writer to be a product of magmatic flow. Part of the northern contact and the southern and western contacts dip inward at moderate angles, whereas the eastern contact is vertical to outward‐dipping. The pluton is inferred to be mushroom‐shaped, and slightly tilted.

The intensity of aureole deformation is low. The aureole has been flexed into concordancy with the pluton roof, and a rudimentary rim fold‐zone is present around lower levels of the pluton. Intrusion of the granitoid is inferred to be primarily by uplift of its roof and depression of its floor.

The highest‐grade contact metamorphic parageneses developed are: cordierite‐K feldspar‐biotite‐quartzite ± andesine ± epidote in metapelites, and ferrohornblende‐biotite‐muscovite‐andesine‐epidote‐quartz in metavolcarenites. No intrusion‐related foliation or lineation was formed.     

燕山造山带后石湖山碱性环状杂岩体的成因与形成过程

后石湖山杂岩体是与垮塌破火山口有关的碱性环状杂岩体, 主要由呈环形分布的碱性火山岩、环状岩墙(斑状石英正长岩)、嵌套的中心复式岩株(晶洞碱长花岗岩和斑状碱长花岗岩)和锥状岩席(石英正长斑岩和花岗斑岩)组成.LA-ICPMS锆石U-Pb年代学分析表明, 斑状石英正长岩环状岩墙、石英正长斑岩和花岗斑岩锥状岩席的侵位年龄分别为119±3Ma、121±2Ma和121±2Ma.该环状杂岩体火山岩与侵入岩的形成年龄相近, 体现了它作为火山-侵入杂岩体的特征.斑状石英正长岩富碱(Na₂O+K₂O=10.0%~10.5%), K₂O含量较高(5.21%~5.42%), 具正的Eu异常(Eu/Eu*=1.05~1.40).碱长花岗岩和斑岩均具有富碱、高FeOtot/MgO、Ga/Al、Zr、Nb和REE值(Eu除外), 以及低Al₂O₃、CaO、MgO、Ba、Sr和Eu含量的特征, 都属于A型花岗岩质岩石.其中斑岩为铝质A型花岗岩, 具有高的初始岩浆温度(880~901℃).所有A型花岗质岩石均具有较富集的Nd同位素组成, ε_Nd(t)值变化于-13.9~-12.2之间.斑状石英正长岩是下地壳中-基性麻粒岩和片麻岩部分熔融产生的熔体与幔源玄武质岩浆混合, 后又发生单斜辉石分离结晶的产物; 碱长花岗岩源于上地壳长英质岩石部分熔融产生的熔体与幔源玄武质岩浆混合, 随后经历长石的分离结晶作用而成; 斑岩是受幔源岩浆底侵加热的上地壳长英质岩石的部分熔融产生的熔体, 并经历了长石的分离结晶作用而产生.该环状杂岩体的形成过程可以概括为: (1)火山爆炸性喷发形成大量的碱性火山熔岩和火山碎屑岩; (2)地下岩浆房空虚导致压力下降, 其顶板围岩失稳而沿火山口周围近直立的环状断裂垮塌, 形成塌陷的破火山口.与此同时, 下覆岩浆房的岩浆被动挤入环状断裂而形成斑状石英正长岩环状岩墙; (3)浅部地壳的长英质岩浆房过压, 促使其高温过碱质A型花岗质岩浆上升侵位形成了中心的斑状碱长花岗岩岩株, 这些岩浆的上涌导致上覆围岩产生倾角中-陡的、内倾的锥状裂隙, 为石英正长斑岩锥状岩席侵位提供了空间; (4)浅部岩浆房复活, 高温过碱质A型花岗质岩浆再度上升侵位形成被嵌套的晶洞碱长花岗岩岩株.同样, 这种岩浆的再度上侵导致上覆围岩产生了倾角较陡而内倾的锥状裂隙, 为花岗斑岩锥状岩席提供了侵位空间.后石湖山碱性环状杂岩体的形成是华北东部早白垩世与克拉通破坏相关的伸展构造体制下的产物, 这种构造体制可能与古太平洋板块的俯冲作用有关.      

Subduction-flip during Iapetus Ocean closure and Baltica–Laurentia collision, Scandinavian Caledonides

Evidence is presented here from the northern Scandinavian Caledonides for development of an extensional basin of Ashgill to Mid Llandovery age along the Baltoscandian margin immediately prior to Baltica–Laurentia collision. U/Pb multigrain and ion microprobe zircon dating of plagiogranites in the Halti Igneous Complex complement previous baddeleyite and zircon dating of a dolerite dyke, and zircon dating of anatectic granite; they demonstrate that this dunite, troctolite, gabbro, sheeted‐dyke complex ranges in age from c. 445 to 435 Ma. The dolerite dykes intruded and melted arkoses of inferred Neoproterozoic age. This evidence, taken together with previous documentation of ophiolites (Solund–Stavfjord), ophiolite‐like associations (Sulitjelma Igneous Complex) and several other mafic suites (e.g. Råna, Artfjället) of Ashgill to Llandovery age further south in the northern Scandinavian Caledonides, implies that Scandian collisional orogeny along this nearly 2000‐km‐long mountain belt was immediately preceeded by development of short‐lived marginal basins. The latter developed during the final closure of the Iapetus Ocean and are inferred to be of back‐arc origin, some (perhaps all) related to E‐dipping subduction. Collision of the continents at c. 435 Ma is inferred to have induced a flip in subduction polarity, leading to underthrusting of Laurentia by Baltica.     

Geochemistry of tertiary alkaline rocks of the Eastern Trans-Pecos Magmatic Province,Texas

The Trans-Pecos Magmatic Province (TPMP) is an alkalic field that was active between 48-17 Ma. Rocks of two subprovinces in the eastern alkalic belt of the TPMP, the Big Bend region and the Davis Mountains, have been analyzed for major and trace element concentrations in order to determine what magmatic processes operated to influence the compositional evolution of the magmas, and to explore what relationship existed between the silica-oversaturated evolved rocks and the silica-undersaturated mafic rocks. Similar compositional trends exist in both subprovinces, implying that the evolved rocks are genetically related to the mafic rocks, and that the differentiation processes were broadly reproducible spatially and temporally.Four stages of evolution have been identified: Stage I, alkali basalt to trachyte; stage II, trachyte to quartz trachyte; stage III, quartz trachyte to rhyolite/comendite; and stage IV, rhyolite/comendite to high-silica rhyolite/comendite. These stages were identified by discontinuities in trends on variation diagrams; within stages I and II, more than one subtrend exist.Stage I can best be modeled as the result of simple crystal fractionation with minor magma replenishment. Two subtrends within this stage indicate that variations in the processes or their rates may have occurred. Stage II exhibits both closed- and open-system behavior. The open system behavior consists of combined fractionation-assimilation and episodic mixing of stage II and stage I magmas. Stages III and IV evolved under open system processes of combined fractionation-assimilation, with the assimilant having compositional characteristics of a shale-dominated sedimentary assemblage.The four stages and subtrends within the stages occur in both subprovinces. Further, some subtrends comprise rocks that differ in age by as much as 10 m.y. In each subprovince, the stratigraphy indicates a random interlaying of rocks of the different stages, generally erupted from more than one center. That contemporaneous magmas of different stages existed in a given subprovince is indicated by the interfingering of their erupted products. These constraints argue against a single magma production-evolution scheme. Rather, the data suggest that magmas of the different stages were produced more than once during the evolution of the eastern TPMP, and that during any given time, production and evolution of magma of all stages were occurring.     

浙江拔茅破火山岩浆作用：开放体系多机制复合演化

拔茅火山杂岩的成分变化范围广泛，包括玄武质，玄武粗安质，粗面安山质，粗面英安质，流纹英安质到流纹质和高硅流纹质岩石都有，它们不是由单一母岩浆演化而来，也不是由单一岩浆房喷发形成的，而是来自多种类型的岩浆房，并经历过复杂的演化过程，为开放体系多机制复合演化，其中早期双峰式基性端元是由上地幔部分熔融形成的，而酸性端无则是地壳物质边熔融边喷发（分离熔融）的结果，作为火山杂岩主体的粗面英安质岩石，其岩浆是在高位主岩浆房内由玄武质岩浆与流纹英安质岩浆混合形成的，发生混合的流纹英安质与玄武质岩浆的比例为57：43，而粗面安山质岩浆则是溶部岩浆房内由玄武质岩浆分离结晶形成的，晚期侵出－侵入的流纹英安质岩穹和高硅流纹斑岩株则分别代表高位次级岩浆房的成分及其硅质帽。     

Petrogenesis and geological implications of the ca.2520Ma gabbroic intrusions in Wutai Mountain of the North China CratoSCIEI北大核心CSCD

本次研究从华北克拉通五台地区基底中识别出一套-25亿年的基性侵入体(主要为岩株)。其中,龙王堂岩株出露面积约5km2,侵入新太古界五台群台怀亚群,岩石类型主要为粗粒辉长岩。一条北东向基性岩墙侵入该岩株,岩墙边部发育明显的冷凝边,岩性为中粒辉长岩。LA-ICP-MS锆石U-Pb同位素年代学分析显示龙王堂粗粒辉长岩岩株形成时代为新太古代晚期(-2520Ma),中粒辉长岩岩墙形成时代为晚三叠世(-230Ma)。锆石原位Lu-Hf同位素分析显示龙王堂岩株具有正的εHf(t)值(4.75-7.95,峰值为6.3),其亏损地幔Hf模式年龄与岩浆年龄接近(tDM=2.64-2.53Ga),指示龙王堂粗粒辉长岩的原始岩浆来自于亏损地幔端元。全岩主微量元素地球化学分析显示龙王堂粗粒辉长岩表现出与洋岛玄武岩(OIB)相似的地球化学特征,并经历了橄榄石和单斜辉石等的结晶分异。结合前人研究成果,本文认为龙王堂新太古代晚期辉长岩岩株与区域上绿岩带(五台群)及同时期花岗侵入体形成时代接近,可能共同指示早期地幔直接分异形成陆壳物质的过程。这可能与俯冲构造体制不同,而与地幔柱作用产生的岩浆特征更为相似。     

Petrogenesis of low-δ<Superscript>18</Superscript>O quartz porphyry dykes,Koegel Fontein complex,South Africa

This paper investigates the origin of low-δ¹⁸O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ¹⁸O values (average 8.0‰, ± 0.7, n?=?33) to the granite (average 8.3?±?1.0, n?=?7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ¹⁸O values (average 1.6‰, ± 2.1, n?=?55). In some cases quartz phenocrysts have δ¹⁸O values as low as ? 2.5‰. The variation in δ¹⁸O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ¹⁸O values have been affected by fluid–rock interaction. Based on a ?_quartz?magma value of 0.6‰, magma δ¹⁸O values must have been as low as ? 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ¹⁸O values that range from +?1.1 to +?4.6‰, and ? 2.3 to +?5.6‰, respectively. These δ¹⁸O values correlate negatively (r = ? 0.96) with initial ⁸⁷Sr/⁸⁶Sr, which can be explained by the event that lowered δ¹⁸O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by ¹⁸O-depleted meteoric water during global glaciation at ~?550 Ma. The early melts had variable δ¹⁸O value but as melt pockets interconnected during melting, the δ¹⁸O values approached that of average gneiss. Variable quartz phenocryst δ¹⁸O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ¹⁸O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.     

AMS studies on a 450 km long 2216 Ma dyke from Dharwar craton,India:Implications to magma flow

Anisotropy of magnetic susceptibility(AMS)studies were carried out on a precisely dated(2216.0±0.9 Ma),450 km long N-S striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke length.In order to use the imbrication of the magnetic foliation,forty eight samples were collected from 13 locations along the length of the dyke.Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite.The corrected anisotropy degree(P_j)of the samples was found to be low to moderate,between 1.007 and 1.072,which indicates primary magnetic fabric.The magnetic ellipsoid is either triaxial,prolate or oblate and clearly defines normal,intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement.The maximum susceptibility axes(K_(max))of the AMS tensor of the dyke is predominantly inclined at low angles(30°),with no systematic variation in depth along the N-S profile,indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source(mantle plume?),flow dynamics together with the compressive stresses exerted by the overlying crust.