Sequence of magmatic events in the Late Paleozoic of Transbaikalia,Russia (U-Pb isotope data)

The Late Paleozoic intrusive rocks, mostly granitoids, totally occupy more than 200,000 km² on the territory of Transbaikalia. Isotopic U-Pb zircon dating (about 30 samples from the most typical plutons) shows that the Late Paleozoic magmatic cycle lasted for 55–60 m.y., from ~330 Ma to ~275 Ma. During this time span, five intrusive suites were emplaced throughout the region. The earliest are high-K calc-alkaline granites (330–310 Ma) making up the Angara–Vitim batholith of 150,000 km² in area. At later stages, formation of geochemically distinct intrusive suites occurred with total or partial overlap in time. In the interval of 305–285 Ma two suites were emplaced: calc-alkaline granitoids with decreased SiO₂ content (the Chivyrkui suite of quartz monzonite and granodiorite) and the Zaza suite comprising transitional from calc-alkaline to alkaline granite and quartz syenite. At the next stage, in the interval of 285–278 Ma the shoshonitic Low Selenga suite made up of monzonite, syenite and alkali rich microgabbro was formed; this suite was followed, with significant overlap in time (281–276 Ma), by emplacement of Early Kunalei suite of alkaline (alkali feldspar) and peralkaline syenite and granite. Concurrent emplacement of distinct plutonic suites suggests simultaneous magma generation at different depth and, possibly, from different sources. Despite complex sequence of formation of Late Paleozoic intrusive suites, a general trend from high-K calc-alkaline to alkaline and peralkaline granitoids, is clearly recognized. New data on the isotopic U-Pb zircon age support the Rb-Sr isotope data suggesting that emplacement of large volumes of peralkaline and alkaline (alkali feldspar) syenites and granites occurred in two separate stages: Early Permian (281–278 Ma) and Late Triassic (230–210 Ma). Large volumes and specific compositions of granitoids suggest that the Late Paleozoic magmatism in Transbaikalia occurred successively in the post-collisional (330–310 Ma), transitional (305–285 Ma) and intraplate (285–275 Ma) setting.     

Collision‐related granite magma genesis,potential sources and tectono‐magmatic evolution: comparison between central,northwestern and western Anatolia (Turkey)

The central, northwestern and western Anatolian magmatic provinces are defined by a large number of late Mesozoic to late Cenozoic collision‐related granitoids. Calc‐alkaline, subalkaline and alkaline intrusive rocks in central Anatolia are mainly metaluminous, shoshonitic, I‐ to A‐types. They cover a petrological range from monzodiorite through quartz monzonite to granite/syenite, and are all enriched in LILE. Their geochemical characteristics are consistent with formation from a subduction‐modified mantle source. Calc‐alkaline plutonic rocks in northwestern Anatolia are mainly metaluminous, medium‐ to high‐K and I‐types. They are monzonite to granite, and all are enriched in LILE and depleted in HFSE, showing features of arc‐related intrusive rocks. Geochemical data reveal that these plutons were derived from partial melting of mafic lower crustal sources. Calc‐alkaline intrusive rocks in western Anatolia are metaluminous, high‐K and I‐types. They have a compositional range from granodiorite to granite, and are enriched in LILE and depleted in HFSE. Geochemical characteristics of these intrusive rocks indicate that they could have originated by the partial melting of mafic lower crustal source rocks.     

Early Paleozoic batholiths in the northern part of the Kuznetsk Alatau: Composition,age, and sources

The paper reports geological, chemical, and geochronological data on the Early Paleozoic granitoid and gabbro-granite associations, which compose the Kozhukhovskii and Dudetskii batholiths in the northern part of the Kuznetsk Alatau. The Kozhukhovskii batholith located in the Alatau volcanoplutonic belt is made up of tholeiitic, calc-alkaline, and subalkaline rocks that were formed in two stages. The first stage corresponded to the formation of granitoids of the Tylinskii quartz diorite-tonalite-plagiogranite complex (～530 Ma, Tylinskii Massif, tholeiitic type) in an island arc setting. The second stage (～500 Ma) produced the Martaiga quartz diorite-tonalite-plagiogranite complex (Kozhukhovskii Massif, calc-alkaline type) and the Krasnokamenskii monzodiorite-syenite-granosyenite complex (Krasnokamenskii Massif, subalkaline type) in an accretionary-collisional setting. The Dudetskii batholith is situated in the Altai-Kuznetsk volcanoplutonic belt and contains widespread subalkaline intrusive rocks (Malodudetskii monzogabbro-monzodiorite-syenite and Karnayul’skii granosyenite-leucogranite complexes) and less abundant alkaline rocks (Verkhnepetropavlovskii carbonatite-bearing alkaline-gabbroid complex), which were formed within the age range of 500–485 Ma. Our Nd isotopic studies suggest mainly a subduction source of the rocks of the Kozhukhovskii batholith (ε_Nd from + 4.8 to + 4.2). Subalkaline rocks of the Dudetskii batholith exhibit wide isotopic variations. The Nd isotopic composition of monzodiorites and monzogabbro of the Malodudetskii Complex (ε_Nd = + 6.6), in association with the elevated alkalinity and high Nb and Ta contents of these rocks, testifies to the predominant contribution of an enriched mantle source at the participation of a depleted mantle source. The lower ε_Nd (from + 3.2 to + 1.9) in its syenites possibly indicates their generation through melting of metabasic rocks derived from enriched mantle protolith. The rocks of the Karnayul’skii Complex have lower Nb and Ta contents at similar ε_Nd (+3.6), which suggests some crustal contribution to their formation.     

Origin of late Archean granite: geochemical evidence from the Vermilion Granitic Complex of northern Minnesota

The 2,700-Ma Vermilion Granitic Complex of northern Minnesota is a granite-migmatite terrane composed of supracrustal metasedimentary rocks, mafic rocks, tonalitic and granodioritic plutonic rocks, and granite. The metasedimentary rocks are predominantly graywacke, which has been regionally metamorphosed to garnet-sillimanite-muscovite-bearing biotite schist, and has locally undergone anatexis. The mafic rocks form early phases within the complex and are of two types: (1) basaltic amphibolite, and (2) monzodiorite and essexite rich in large ion lithophile elements (LILE). The members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks, producing an early migmatite. The granite is of two distinct varieties: (1) white garnet-muscovite-biotite leucogranite (S-type; Chappell and White 1974) and (2) grayish-pink biotite-magnetite Lac La Croix Granite (I-type). The leucogranite occurs in the early migmatite and in paragneissic portions of the complex, whereas the Lac La Croix Granite is a late-stage intrusive phase that invades the early migmatite and metasediment (producing a late migmatite) and forms a batholith. This study focuses specifically on the origin of granite in the Vermilion Granitic Complex. Chemical mass-balance calculations suggest that the S-type two-mica leucogranite had a metagraywacke source, and that the I-type Lac La Croix Granite formed via partial fusion of calc-alkaline tonalitic material, which may have been similar to rocks of the early plutonic suite. This model is satisfactory for petrogenesis of similar Late Archean post-kinematic granites throughout the Canadian Shield.     

Petrogenesis of collision-related plutonics in Central Anatolia, Turkey

Central Anatolia exhibits good examples of calc-alkaline and alkaline magmatism of similar age in a collision-related tectonic setting (continent–island arc collision). In the Central Anatolia region, late Cretaceous post-collisional plutonic rocks intrude Palaeozoic–Mesozoic metamorphic rocks overthrust by Upper Cretaceous ophiolitic units to make up the Central Anatolian Crystalline Complex.

In the complex, three different intrusive rock types may be recognised based on their geochemical characteristics: (i) calc-alkaline (Behrekdag, Cefalikdag, and Celebi); (ii) subalkaline-transitional (Baranadag); and (ii) alkaline (Hamit). The calc-alkaline and subalkaline plutonic rocks are metaluminous I-type plutons ranging from monzodiorite to granite. The alkaline plutonic rocks are metaluminous to peralkaline plutons, predominantly A-type, ranging from nepheline monzosyenite to quartz syenite.

All intrusive rocks show enrichment in LILE and LREE relative to HFSE, and have high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd ratios. These characteristics indicate an enriched mantle source region(s) carrying a subduction component inherited from pre-collision subduction events. The tectonic discrimination diagram of Rb vs. (Y+Nb) suggests that the calc-alkaline, subalkaline, and alkaline plutonic rocks have been affected by crustal assimilation combined with fractional crystallisation processes.

The coexistence of calc-alkaline and alkaline magmatism in the Central Anatolian Crystalline Complex may be attributed to mantle source heterogeneity before collision. The former carries a smaller intraplate component and pre-subduction enrichment compared to the latter. Either thermal perturbation of the metasomatised lithosphere by delamination of the thermal boundary layer (TBL), or removal of a subducted plate (slab breakoff) is the likely mechanism for the initiation of the post-collisional magmatism in the Complex.     

Late Paleozoic granitoids in western Transbaikalia: sequence of formation,sources of magmas,and geodynamics

The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.     

Mantle and Crustal Sources in the Genesis of Late-Hercynian Granitoids (NW Portugal): Geochemical and Sr-Nd Isotopic Constraints

Large volumes of granitoids were emplaced in the Hercynian Central Iberian Zone during the last ductile deformation phase (D3, 300-320 Ma). The biotite-rich granitoids are the most abundant: (1) syn-D3 granodiorites-monzogranites (313-319 Ma) with calc-alkaline and aluminopotassic affinities; (2) late-D3 granodiorites-monzogranites (306-311 Ma), related to subalkaline and aluminopotassic series. These granitoids are associated with coeval gabbro-norite to granodiorite bodies and/or mafic microgranular enclaves. Both granitoids and basic-intermediate rocks show petrological, geochemical and isotopic evidence of interaction between felsic and mafic magmas.The mantle-derived melts, represented by shoshonitic gabbro-norites, were probably derived from an enriched and isotopically homogeneous source (Sr_i = 0.7049 to 0.7053, e_Nd = -2.1 to -2.5). In some syn- and late-D3 plutons there are evidences of essentially crustal granites, represented by moderately peraluminous monzogranites of aluminopotassic affinity. They have similar Nd model ages (1.4 Ga) but different isotopic compositions (Sr_i = 0.7089 to 0.7106, e_Nd = -5.6 to -6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or felsic meta-igneous lower crust materials. Large amounts of hybrid magmas were generated by the interaction of these coeval mantle- and crust-derived liquids, giving rise to slightly peraluminous monzogranites/granodiorites of calc-alkaline and subalkaline affinities, which display more depleted isotopic compositions than the crustal end-members (Sr_i = 0.7064 to 0.7085, e_Nd = -4.4 to -6.2). Petrogenetic processes involving mingling and/or mixing and fractional crystallization (at variable degrees) in multiple reservoirs are suggested.A major crustal growth event occurred in late-Hercynian times (∼305-320 Ma) related to the input of juvenile mantle magmas and leading to the genesis of composite calc-alkaline and subalkaline plutons, largely represented in the Central Iberian Zone.     

High-K, calc-alkaline I-type granitoids from the composite Yozgat batholith generated in a post-collisional setting following continent-oceanic island arc collision in central Anatolia, Turkey

Summary The composite Yozgat batholith consists of a S-I-A-type granitoid association intruding the supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and medium- to high-grade metasedimentary rocks of the central Anatolian crystalline complex. These rocks are unconformably covered by Palaeocene to Early Eocene sedimentary rocks. The I-type granitoids are the most common rock association of this huge batholith. In an area between the towns of Şefaatli and Yerk?y, the southwestern part of the batholith can be subdivided into five mappable units: the Ak?akoyunlu quartz monzodiorite (mafic; hornblende K-Ar cooling ages of 77.6–79.3 Ma); the Cankılı monzodiorite (mafic; hornblende K-Ar cooling age of 71.1 Ma); the Adatepe quartz monzonite (mafic; hornblende K-Ar cooling age of 68.0 Ma); the Yassıağıl monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 69.9–79.8 Ma) and the Karakaya monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 71.3–77.0 Ma). All the lithological units, except the Karakaya monzogranite, include large K-feldspar megacrysts and various types of mafic microgranular enclaves in field outcrops, indicating mingling and mixing. In addition, microscopic textures showing the hybridization between the coeval mafic and felsic magma sources are present. Whole-rock major element geochemistry shows a high-K calc-alkaline, metaluminous, I-type composition with an aluminium saturation index (ASI) less than 1.10 and with CIPW diopside content in all the lithological units. Large ion lithophile elements (LILE), light rare earth elements (LREE), some high field strength elements (HFSE) (except Nb) enrichments and significant crustal contribution revealed by the oxygen and sulphur stable isotope compositions in the mafic and felsic I-type granitoid units are consistent with mafic lower crustal and metasomatized mantle sources the latter of which were metasomatized by earlier supra subduction zone (SSZ)-derived fluids during the development of the SSZ-type central Anatolian ophiolite. Supplementary material to this paper is available in electronic form at     

Granites of western Karakorum and northern Kohistan (Pakistan): A composite mid-cretaceous to upper cenozoic magmatism

The petrography, the chemical-mineralogical typology and the ages of six plutonic units, four from the Karakorum axial batholith (Darkot Pass, Ghamu Bar, Batura and Hunza units) and two of northern Kohistan (Gindai and Nomal plutons) are defined and compared based on field data and analyses of up to 78 samples (major elements, REE, Rb-S.sbnd;Sr and KAr isotopes). The Karakorum axial batholith is a composite body. Three major intrusive stages occurred: (1) around Mid-Cretaceous times (ca. 110-95 Ma) with the emplacement of subalkaline, i.e. monzonitic (Darkot Pass) and calc-alkaline (Hunza, ?Ghamu Bar) units; (2) during Palaeogene, maybe up to 43 Ma (Batura subalkaline unit). A strong tectonommetamorphic event, recorded in the gneissification of the Cretaceous intrusives, occurred between these two stages; it may be of Palaeocene age. P-T estimates of the highest metamorphic grade rocks of the Hunza unit have yielded values of 580–640°C and 5 ± 0.5 kbar; and (3) during Upper Miocene (ca. 9 Ma; Baltoro subalkaline unit; Debon et al., 1986c). In addition, a conspicuous network of aplo-pegmatitic dykes emplaced into the Hunza area, possibly from the Eocene up to the Upper Cenozoic with a maximum during the Middle Miocene (ca. 15 Ma). Most of these major magmatic stages are met again among the acidic intrusives and dykes of northern Kohistan: the first one as blastomylonitic tholeiitic plutons (Nomal; ca. 102 Ma; Petterson and Windley, 1985), the second one as subalkaline plutons (Gindai; ca. 59 Ma), and a third one as leucocratic dykes, of Oligocene age (ca. 30 Ma; Petterson and Windley, 1985).These data may be related to the geodynamic evolution of the NW part of the India-Eurasia suture zone, thus allowing better constraints on the major steps of this evolution. The partly synchronous closures, by N-dipping subduction, of the two Tethys branches which are assumed to have encircled the Kohistan arc in Upper Mesozoic times, may have generated both the Karakorum and the Kohistan intrusives of Cretaceous and Palaeogene ages. The northern branch very likely closed before the southern one. At the time of the second intrusive stage (Palaeogene): (a) Kohistan and Karakorum had already collided, were welded and had suffered the same major tectonometamorphic event; (b) subduction of the southern Tethys floor beneath the welded Kohistan-Karakorum was still active; (c) however, collision between India and Kohistan-Karakorum may have already begun, particularly at the level of the Nanga Parbat promontory. Finally, it is emphasized that the intrusive processes continued in the Karakorum long after the collision (e.g., Baltoro granite).     

http://www.sciencedirect.com/science/article/pii/S1674987113001345

Many elongated, lenticular plutons of porphyritic granitoids are distributed mainly near the southern and northern margin of the Chhotanagpur Gneissic Complex （CGC） which belongs to the EW to ENE-WSW tending 1500 km long Proterozoic orogenic belt amalgamat ng the North and South Indian cratonic blocks. The late Grenvillian （1071 ±64 Ma） Raghunathpur porphyritic granitoid gneiss （PGG） batholith comprising alkali feldspar granite, granite, granodiorite, tonalite, quartz syenite and quartz monzonite intruded into the granitoid gneisses of southeastern part of CGC in the Purulia district, West Bengal and is aligned with ENE-WSW trending North Purulia sr~ear zone, Mineral chemistry, geochemistry, physical condition of crystallization and petrogenetic model of Raghunathpur PGG have been discussed for the first time. The petrographic and geochemical features （including major and trace- elements, mineral chemistry and S7Sr/S6Sr ratio） suggest these granitoids to be classified as the shosh- onitic type. Raghunathpur batholith was emplaced at around 800 ~C and at 6 kbar pressure tectonic discrimination diagrams reveal a post-collision tectonic setting while structural studies reveal its emplacement in the extensional fissure of North Purulia shear zone. l＇he Raghunathpur granitoid is compared with some similar granitoids of Europe and China to draw its petrogenetic model. Hybridi- zation of mantle-generated enriched mafic magma and crustal magma at lower crust and later fractional crystallization is proposed for the petrogenesis of this PGG. Mafic magma generated in a post-collisional extension possibly because of delamination of subducting slab. Raghunathpur batholith had emplaced in the CGC during the final amalgamation （~ 1.0 Ga） of the North and South Indian cratonic blocks. Granitoid magma, after its generation at depth, was transported to its present level along megadyke channel, ways within shear zones.     

Geochemical exploration of a Precambrian Batholith, source of a Cu-W mineralization of the tourmaline breccia in Southern Finland

The Hämeenkyrö batholith is a round-shaped plutonic body of an areal size of 147 km². It is composed of calc-alkaline to alkaline rocks that intruded previously metamorphosed Svecofennian volcanogenic and sedimentary schists 1860 Ma ago. The Cu-W bearing tourmaline breccia of the Ylörvi deposit occurs in metavolcanic rocks close to the eastern contact of the batholith.The average sampling density in the batholith was 1 sample per km², and 175 samples were analyzed for Cu, Au, Ag, Ni, Pb, Co, Zn, S by AAS for SiO₂, TiO₂, Al₂O₃, FeO, MnO, MgO, CaO, Na₂O, K₂O, As, Sn and P by X-ray fluorescence. Mo and W were determined colorimetrically. Barth mesonorms were calculated for each sample and the rock type was determined according to Streckeisen's classification. Element distributions are displayed on contour maps.The rock types of the batholith exhibit an asymmetric concentric arrangement, the order from the center towards the margin being alkali-feldspar granite, syenogranite, monzogranite, quartz monzonite, quartz syenite, alkali-feldspar, quartz syenite, syenite and alkali-feldspar syenite. Anomalously high Cu, As, Sn, S, K₂O and Na₂O contents have been found at the eastern margin of the batholith in a N—S-trending zone, which is characterized by hydrothermal alteration phenomena, propylitization, tourmalinization and scapolitization. Three anomalous areas have been defined within this zone, one of them is associated with the Ylöjärvi deposit and the other two are regarded as exploration targets.     

Oxygen Isotopic Compositions of the Cape Ashizuri Granitoids in Southwest Japan

Oxygen isotopic composition was determined on representative samples of the Cape Ashizuri plutonic rocks, in order to estimate the genetic background of the biotite granite and alkaline granitoids. The biotite granite (70.1–76.1% SiO₂) ranged from 8.61 to 9.30‰ δ¹⁸O and averaged as 8.9‰ δ¹⁸O (n = 3), which is much smaller than the same Miocene granitoids of the Okueyama (avg. 10.1‰) and Takakumayama (11.6‰) granitic bodies, which are associated with tin mineralization. Among the alkaline granitoids, quartz syenites also have values as low as 7.14–8.70‰, with an average of 8.0‰ (n = 3), and monzonite and gabbro vary from 6.14 to 7.86‰, with an average of 7.0‰ (n = 3). These alkaline granitoids may be lower crustal in origin. The gabbroids containing 12.5% MgO at the maximum with low Sr initial ratio, are considered to be derived from the upper mantle through the fore‐arc tectonic break‐up on the subducting slab.     

A 2.5 G.a. reworked sialic crust: Rb-Sr ages and isotopic geochemistry of late archaean volcanic and plutonic rocks from E. Finland

In east-central Finland, Archaean terrains present three main lithologic units: a) gneissic basement, emplaced from 2.86 G.a. to 2.62 G.a., b) greenstone belt (2.65 G.a.) and c) calc-alkaline magmatism (2.50 G.a. to 2.40 G.a). Twenty three rocks of the calc-alkaline suite have been chosen for geochronologic and Rb-Sr isotopic studies. These rocks are subdivided into three groups: 1) acid volcanics from Luoma, 2) augen gneiss from Arola, and 3) post kinematik pink leucogranite from Arola. The 2.50±0.10 G.a. age of the Luoma volcanics indicates that they represent the upper part of a greenstone belt composed of a single sequence of volcanic rocks. The ages, initial ⁸⁷Sr/⁸⁶Sr (I_Sr) and major element compositions of the augen gneisses of Arola and Suomussalmi indicate that these rocks are the plutonic equivalents of the Luoma acid volcanics. The Arola pink leucogranite marks the terminal phase of Archaean magmatic activity (from 2.86 G.a. to 2.41 G.a.). This was followed by at least 0.40 G.a. of quiescence. The I_Sr and major element compositions suggest that the genesis of the calc-alkaline magmatic rocks involved crustal materials, but all their geochemical features cannot be explained without the participation of mafic greenstone belt materials. The first crustal components had low I and low K₂O/ Na₂O ratios while the younger ones (calc-alkaline magmas) had medium to high I_Sr and high K₂O/Na₂O ratios. Thus the petrogenetic processes have changed with time from ensimatic to ensialic, implying major reworking of preexisting crustal materials. This evolution leads to the accretion of the continental crust from the mantle.     

义敦岛弧措交玛岩体岩浆混合成因:镁铁质微粒包体的证据

义敦岛弧北部的措交玛岩基岩体主要由黑云母二长花岗岩和边部的花岗闪长岩组成。在黑云母二长花岗岩中存在有少量镁铁质微粒包体,其成分为闪长质,与寄主岩石接触关系从渐变到截然。在包体周围的寄主岩石中存在黑云母、角闪石自身的包含结构,角闪石包含黑云母,斜长石发育明显的溶蚀结构,核部斜长石被溶蚀成筛状,边部环带状斜长石溶蚀不明显,是基性岩浆注入到酸性岩浆中导致岩浆混合的结果。黑云母二长花岗岩具有更高的轻重稀土分异系数,闪长质包体轻重稀土分异系数较低,黑云母二长花岗岩和暗色闪长质微粒包体具有明显相似性的微量元素特征。寄主岩黑云母二长花岗岩锆石U-Pb年龄为236±1.9Ma,闪长质包体为235±3.9Ma,二者形成年代在误差范围内基本一致,可能为甘孜-理塘洋向西俯冲过程中,俯冲洋壳部分熔融形成的玄武质岩浆上涌底侵于壳-幔边界导致地壳的部分熔融形成酸性的黑云母二长花岗岩岩基。     

Temporal and chemical connections between plutons and ignimbrites from the Mount Princeton magmatic center

The Mount Princeton magmatic center, located in central Colorado, consists of the epizonal Mount Princeton batholith, the nested Mount Aetna caldera, and volumetrically minor leucogranites. New CA-TIMS U/Pb zircon ages indicate the majority of the Mount Princeton batholith was emplaced during a period of regional ignimbrite quiescence. The structurally highest unit of quartz monzonite yields a ²⁰⁶Pb/²³⁸U age of 35.80 ± 0.10 Ma, and the youngest dated unit of the quartz monzonite is a porphyritic unit that yields a ²⁰⁶Pb/²³⁸U age of 35.37 ± 0.10 Ma. Using the exposed, dated volume of the quartz monzonite and new geochronology yields an estimated pluton filling rate of ~0.002 km³/a. This rate is comparable to the accumulation rates published for other plutons, and at least an order of magnitude slower than fluxes necessary to support accumulation of large eruptible magma volumes. Geochronology for the two large ignimbrites spatially associated with the batholith indicates a temporal disconnect between the vast majority of pluton building and explosive eruption of magma. The Wall Mountain Tuff erupted from a source in the same geographic area as the Mount Princeton batholith at 37.3 Ma (Ar/Ar sanidine), but no structural evidence of a caldera or temporally associated plutonic rocks is known. The Badger Creek Tuff erupted at 34.3 Ma (Ar/Ar sanidine) during the formation of the Mount Aetna caldera in the southern portion of the batholith. Our ²⁰⁶Pb/²³⁸U age for the Badger Creek Tuff is 34.47 ± 0.05. The only analyzed plutonic rocks of similar age to the Badger Creek Tuff are an extra-caldera dike with a ²⁰⁶Pb/²³⁸U age of 34.57 ± 0.08 Ma, a ring dike with a ²⁰⁶Pb/²³⁸U age of 34.48 ± 0.09 Ma, and a portion of the Mount Aetna pluton with a ²⁰⁶Pb/²³⁸U age of 34.60 ± 0.13 Ma. The small volume intrusions related to the eruption of the Badger Creek Tuff are chemically similar to the ignimbrite and show no signature of crystal–liquid separation in the shallow crust.     

The Northeast Kingdom batholith,Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<₂ (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<₂O/Na₂O (<1), K/Rb (60–350), Zr/Hf (30–50), Nb/Ta (2–22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5–22), Th/Yb (0–27), Ba/La (0–80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Yb_cn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Yb_cn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.     

Origin and evolution of post-collisional magmatism: Coeval Neoproterozoic calc-alkaline and alkaline suites of the Sinai Peninsula

Two Late Neoproterozoic post-collisional igneous suites, calc-alkaline (CA) and alkaline–peralkaline (Alk), widely occur in the northernmost part of the Arabian–Nubian Shield. In Sinai (Egypt) and southern Israel they occupy up to 80% of the exposed basement. Recently published U–Pb zircon geochronology indicates a prolonged and partially overlapping CA and Alk magmatism at 635–590 Ma and 608–580 Ma, respectively. Nevertheless in each particular locality CA granitoids always preceded Alk plutons. CA and Alk igneous rocks have distinct chemical compositions, but felsic and mafic rocks in general and granitoids from the two suites in particular cannot be distinguished by their Nd, Sr and O isotope ratios. Both suites are characterized by positive ε_Nd(T) values, from + 1.5 to + 6.0 (150 samples, 28 of them are new analyses), but predominance of juvenile crust in the region prevents unambiguous petrogenetic interpretation of the isotope data. Comparison of geochemical traits of felsic and mafic rocks in each suite suggests a significant contribution of mantle-derived components to the silicic magmas. Model calculation shows that the alkaline granite magma could have been produced by partial (~ 20%) melting of rocks corresponding to K-rich basalts. Material balance further suggests that granodiorite and quartz monzonite magmas of the CA suite could form by mixing of the granite and gabbro end-members at proportions of 85/15. In the Alk suite, alkali feldspar and peralkaline granites have evolved mainly by fractional crystallization of feldspars and a small amount of mafic minerals from a parental syenogranite melt. Thus the protracted, 20 m.y. long, contemporaneous CA and Alk magmatism in the northern ANS requires concurrent tapping of two distinct mantle sources. Coeval emplacement of CA and Alk intrusive suites was described in a number of regions throughout the world.     

Petrology and petrogenesis of the Mount Stuart batholith — Plutonic equivalent of the high-alumina basalt association?

The Mount Stuart batholith is a Late Cretaceous calc-alkaline pluton composed of rocks ranging in composition from two-pyroxene gabbro to granite. Quartz diorite is most abundant. This batholith may represent the plutonic counterpart of the high-alumina basalt association. A petrogenetic model is developed in which this intrusive series evolved from one batch of magnesian high-alumina basalt, represented by the oldest intrusive phase, by successive crystal fractionation of ascending residual magma. However, the possibility that this intrusive suite originated from an andésite (quartz diorite) parent by fractionation cannot be excluded.Computer modeling of this intrusive sequence provides a quantitative evaluation of the sequential change of magma composition. These calculations clearly indicate that the igneous suite is consanguineous, and that subtraction of early-formed crystals from the oldest rock is capable of reproducing the entire magma series with a remainder of 2–3% granitic liquid. This model requires that large amounts of gabbroic cumulate remain hidden at depth- an amount equal to approximately 8–10X the volume of the exposed batholith. Mass balances between the amounts of cumulate and residual liquid calculated compare favorably with the observed amounts of intermediate rocks exposed in the batholith, but not with the mafic rocks.Mafic magmas probably fractionated at depth by crystal settling, whereas younger quartz diorite and more granitic magmas underwent inward crystallization producing gradationally zoned plutons exposed at present erosional levels.     

U–Pb zircon and monazite geochronology of post-collisional Hercynian granitoids from the Central Iberian Zone (Northern Portugal)

In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U–Pb zircon and monazite geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313±2 Ma, 319±4 Ma, 316±2 Ma and 314±2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309±3 Ma for the Braga granite, 309±1 Ma for the Gonça granite and 311±5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeirós plutons (307±3.5 Ma and 306±2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300±1 Ma), followed by the emplacement of the Peneda–Gerês massif (Gerês, Paufito, Illa and Carris granites). The Gerês granite, emplaced at 296±2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290±2.5 Ma. For the Carris granite a minimum emplacement age of 280±5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313–319 Ma; (2) late-D3 biotite-dominant granitoids, 306–311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290–296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300–320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D4) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290–296 Ma.     

Geochemistry and petrogenesis of post-collisional alkaline and peralkaline granites of the Arabian-Nubian Shield: a case study from the southern tip of Sinai Peninsula,Egypt

The southern Sinai Peninsula, underlain by the northernmost extension of the Arabian-Nubian Shield, exposes post-collisional calc-alkaline and alkaline granites that represent the youngest phase of late Neoproterozoic igneous activity. We report a petrographic, mineralogical and geochemical investigation of post-collisional plutons of alkaline and, in some cases, peralkaline granite. These granites intrude metamorphosed country rocks as well as syn- and post-collisional calc-alkaline granitoids. The alkaline and peralkaline granites of the southern tip of Sinai divide into three subgroups: syenogranite, alkali feldspar granite and riebeckite granite. The rocks of these subgroups essentially consist of alkali feldspar and quartz with variable amounts of plagioclase and mafic minerals. The syenogranite and alkali feldspar granite contain small amounts of calcic amphibole and biotite, often less than 3%, while the riebeckite granite is distinguished by sodic amphibole (5–10%). These plutons have geochemical signatures typical of post-collisional A-type granites and were most likely emplaced during a transition between orogenic and anorogenic settings. The parental mafic magma may be linked to lithospheric delamination and upwelling of asthenospheric mantle material. Differentiation of the underplated basaltic magma with contributions from the juvenile crust eventually yielded the post-collisional alkaline granites. Petrogenetic modelling of the studied granitic suite shows that pure fractional crystallization cannot quantitatively explain chemical variations with the observed suite, with both major oxides and several trace elements displaying trends opposite to those required by the equilibrium phase assemblage. Instead, we show that compositional variation from syenogranite through alkali feldspar granite to riebeckite granite is dominated by mixing between a low-SiO₂ liquid as primitive or more primitive than the lowest-SiO₂ syenogranite and an evolved, high-SiO₂ liquid that might be a high-degree partial melt of lower crust.