Fractional Crystallization Of Mantle-derived Magma Research Articles

Chronological framework of Precambrian Dantazi Complex: Implications for the formation and evolution of the northern North China Craton

The Dantazi Complex in the North Hebei Terrane is located in the middle segment of the northern North China Craton, and plays a crucial role for understanding the early Precambrian crustal evolution of the craton. The Dantazi Complex is composed mainly of late Neoarchean tonalites-trondhjemites-granodiorites, potassic granites, sanukitoid granodiorites-monzodiorites and quartz syenites, which were emplaced by Paleoproterozoic gabbroic stocks. SHRIMP and LA–ICP–MS zircon dating analyses have revealed that the Neoarchean diversified granitoids of the Dantazi Complex were formed in ∼2.52–2.47 Ga, and preserved ∼2.60–2.53 Ga inherited/captured zircon ages. All these rocks experienced early metamorphism and deformation at ∼2.40 Ga, later granulite facies metamorphism at ∼1.80 Ga, and alterations at ∼1.73 Ga. Whole-rock chemical characteristics and Sm–Nd–Lu–Hf isotopes indicate that (1) The ∼2.52 Ga tonalites-trondhjemites-granodiorites were derived from partial melting of juvenile mafic crust; (2) the ∼2.51 Ga sanukitoid monzodiorite-granodiorite series came from slab-mantle interaction with different melt/rock ratios; (3) The ∼2.50–2.47 Ga potassic granites were generated by intracrustal recycling of metagreywacks that sourced from the mixture between juvenile and ancient crustal materials; and (4) The ∼2.50 Ga quartz syenites were originated from fractional crystallization of mantle-derived magmas. These petrogenetic processes reveal that the subduction-related crust-mantle interaction is the key reason for the diversity of late Neoarchean granitoids in the Dantazi Complex. Regional comparison indicates that the Precambrian North Hebei Terrane show distinct structure in the chronology and lithological assemblages from other basement terranes in the trans-North China Orogen and Western Block, implying that they were formed in different crust-mantle dynamic systems. Therefore, the Precambrian North Hebei Terrane was probably an independent micro-continental block at least until the early Paleoproterozoic. In the Archean crystalline basement range of the North Hebei–West Liaoning–East Hebei, from NW to SE, the magmatic crystallization ages gradually became older, and the granitoid assemblages transformed from sodium-rich to potassium-rich, the spatio-temporal evolution of chemical composition and lithological assemblages further indicates that the North Hebei Terrane was most likely developed in an arc environment with continuous retreat subduction caused by the slab roll-back.

Neoproterozoic and Early Paleozoic magmatism in the eastern Lhasa terrane: Implications for Andean-type orogeny along the northern margin of Rodinia and Gondwana

Precambrian and Early Paleozoic magmatic activities in the Lhasa terrane provides important context for distinguishing between the North and South Lhasa terranes, and deciphering the nature and geotectonic history of the northern margin of the Rodinia and Gondwana supercontinents. Here, we conducted a detailed petrological, geochemical, and geochronological study of mafic to granitic meta-intrusive rocks of the Bomi complex in the eastern segment of the Lhasa terrane. Zircon U–Pb dating indicates that these rocks record Neoproterozoic and Cambrian magmatic events, with protolith crystallization ages of ca. 805 Ma, ca. 505 Ma, and ca. 503 Ma for dioritic gneisses, amphibolites (meta-gabbros), and granitic gneisses, respectively. These rocks were later metamorphosed during the Late Jurassic, with zircon U–Pb ages of ca. 158 Ma obtained from the amphibolites. All rocks are enriched in Rb, U and LREEs, and depleted in Nb, Ta, Sr and HREEs, showing affinity with arc-related magmas. With positive zircon εHf(t) values of +1.4 to +6.9, the Neoproterozoic dioritic gneisses are interpreted to have formed by fractional crystallization of mantle-derived magmas. Cambrian amphibolites have εHf(t) values of –3.0 to +6.0, differing from those of coeval granitic gneisses (+0.3 to +3.2). These amphibolites are interpreted to have been derived from partial melting of enriched continental lithospheric mantle with contamination of subducted sediments, whereas the granitic gneisses probably originated from anatexis of lower crust that was induced by coeval emplacement of mantle-derived magma. These rocks are interpreted to be formed in a continental arc setting. When comparing these data with those of other Precambrian and Cambrian magmatic rocks in the Lhasa terrane and other blocks, we propose that the North Lhasa terrane and South Lhasa terrane have distinctive Precambrian basements. The North Lhasa terrane diachronically witnessed an Andean-type orogeny along the northwestern margin of Rodinia during the Neoproterozoic and the northeastern margin of Gondwana during the Early Paleozoic.

Geochemical and thermodynamic modeling of the petrogenesis of A1-type granites and associated intermediate rocks: A case study from the central Fennoscandian Shield

The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive εNd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average εNd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The εNd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.

Geochronology and petrogenesis of paleoproterozoic post-collisional quartz monzodiorites from the Helanshan Complex, North China Craton: Implications for crust–mantle interaction

Insights into crust–mantle interaction in continental collisional orogens is key to understanding high- and ultrahigh-temperature (HT–UHT) metamorphism, continental crustal growth, and the tectonic evolution of such orogens. Here, we present whole-rock major- and trace-element and Nd isotope geochemistry, as well as zircon U–Pb geochronology and Hf isotope data, for quartz monzodiorites from the Helanshan Complex to constrain their petrogenesis and draw implications regarding crust–mantle interaction during formation of the Khondalite Belt in the North China Craton. LA–ICP–MS zircon U–Pb dating yields crystallization ages of 1938–1931 Ma for three samples, contemporaneous with spinel-bearing UHT pelitic granulites from the Helanshan Complex. The quartz monzodiorite samples have variable whole-rock compositions, with relatively low SiO2 (low to 51.41 wt.%) but high MgO (up to 5.05 wt.%) and Cr (up to 80.9 ppm) contents, as well as high Mg# values (up to 0.50), which suggest their derivation from the mantle. These samples exhibit a narrow range of εNd(t) values of −0.3 to + 1.0, with no decrease as decreasing whole-rock MgO content, and variable zircon εHf(t) values of −2.4 to + 5.8. In addition, Mg# values of hornblende and biotite, and Xan values of plagioclase decrease gradually with increasing whole-rock SiO2 content. All of these geochemical data indicate that the studied quartz monzodiorites were produced by fractional crystallization of mantle-derived magma, with assimilation of crustal material. On the basis of previously published data in the Khondalite Belt, the Helanshan quartz monzodiorites formed in the post-collisional setting, and record a tectonic transition from compression to extension during the period ca. 1.95–1.93 Ga. The emplacement of mantle-derived magma that formed the quartz monzodiorites through differentiation at ca. 1.93 Ga, was possibly responsible for the formation of ca. 1.93 Ga UHT rocks in the Helanshan Complex.

Cretaceous volcanic-plutonic magmatism in SE China and a genetic model

Intensive and extensive Cretaceous volcanic-plutonic magmatism occurred in the coastal area of SE China, which is an important part of circle Pacific continental margin associated with copper and gold metallogenesis. Representative calderas are studied in detail, which show “coeval”, “cogenetic” and “complementary” geochemical and petrological signatures for the eruptive and intrusive units. Contemporary abyssal plutonic associations of gabbro-diorite-granite are revisited for their micropetrographical and geochemical fingerprints, which show distinct compaction-driven fractionation of plagioclase and amphibole for the gabbroic rocks. Microtextures and trace elements are also analyzed for underplating/intraplating gabbroic rocks in the lower crust, which were carried out by Cenozoic basalt as deep-seated xenoliths, to test accumulation and fractionation of the mantle-derived magma. A dataset of geochemistry for the Cretaceous volcanic-plutonic rocks is compiled from previous and this studies. Comparisons with magmatism of typical extensional continental arc (northern Cascades in North America), compressional continental arc (Andes in South America) and rifted arc (Taupo in New Zealand) show that the Cretaceous magmatism in the coastal area of SE China is more approximate to the rifted arc one and dominated by felsic magmatism. A series of numerical modeling using major and trace elements and isotopes, as well as petrographic evidence reveal that a combination of fractional crystallization of mantle-derived magma, partial melting of pre-existed mid-lower crust and magma mixing played an important role in the compositional variations of the Cretaceous volcanic-plutonic rocks in SE China.By integrating petrology, geochemistry and geophysical data, we propose a trans-crustal magma system model for the Cretaceous volcanic-plutonic magmatism in SE China. In this model, significant fractionation combining distinct contribution of crustal materials can interpret the dominant felsic component feature. Therefore, the Cretaceous volcanic-plutonic rocks in SE China recorded the evolution of a continental arc built on a complex trans-crustal magmatic system.

Intraplate adakite-like rocks formed by differentiation of mantle-derived mafic magmas: A case study of Eocene intermediate-felsic porphyries in the Machangqing porphyry Cu-Au mining district, SE Tibetan plateau

Intraplate intermediate-felsic adakite-like rocks are widely considered to be generated via partial melting of the mafic lower crust. However, the genesis of phenocrysts of mafic minerals from some adakite-like rocks suggests that they can originate from mantle-derived magmas through assimilation and fractional crystallization (AFC) processes. In this study, we investigated the petrogenesis of three phases of Eocene porphyries, namely quartz diorite porphyry (QDP-I and QDP-II), granodiorite porphyry (GDP), and granite porphyry (GP), exposed in the Machangqing area of the western Yangtze Craton in the southeastern Tibetan plateau. These porphyries were formed at 34.2–35.2 Ma in an intracontinental extension setting and present adakitic affinities of high Sr/Y (50–95) and La/Yb (50–113) ratios. The similar characteristics of these Machangqing porphyries in terms of whole-rock trace elements and Sr–Nd isotopes, as well as in the mineralogy of amphibole phenocrysts and clinopyroxene antecrysts, indicate that these intermediate-felsic porphyries were derived from the same source. The important observation for investigating the parental magma of these intermediate-felsic porphyries is that they have close temporal and spatial correlations with mantle-derived mafic volcanic rocks. These intermediate-felsic porphyries have slightly enriched Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7068–0.7077, εNd(t) = −5.1 to −6.6], affected by limited crustal assimilation, as compared to those of contemporaneous mafic volcanic rocks. The high-Al amphibole phenocrysts and clinopyroxene antecrysts in QDP-I have similar rare earth element (REE) patterns with clinopyroxene phenocrysts in the mafic volcanic rocks, which suggest that these rocks originated from the same mantle-derived mafic magma. In addition, the major elements of these intermediate-felsic and mafic igneous rocks in the Machangqing area show fractional crystallization trends with an increasing magma silica component. Simulations of the major and trace elements of these igneous rocks show that the sequence of fractional crystallization of olivine, clinopyroxene, garnet, amphiboles, biotite, feldspar, and apatite with time promoted the evolution of the magma series. As revealed by the calculated physicochemical conditions during crystallization of the phenocrysts, the initial mafic magmas were derived from depths of greater than 64 km through the partial melting of the metasomatic lithosphere mantle induced by Eocene intraplate extension in the western Yangtze Craton. These initial mafic magmas underwent three stages of fractional crystallization at three levels of the thickened crust: the first at the base of the lower crust (49–55 km); followed by fractional crystallization at the middle crust (24–32 km); and finally at the upper crust (4–5 km). These fractional crystallization processes caused the initial mafic magmas to evolve through the intermediate (roughly represented by QDP-I) to the felsic compositions (GDP and GP). Whole-rock and mineral geochemistry evidence demonstrates that fractional crystallization of mantle-derived magmas, with a small amount of crustal assimilation, generated these Eocene intraplate intermediate-felsic adakitic porphyries in the Machangqing area.

Differentiation in impact melt sheets as a mechanism to produce evolved magmas on Mars

Asteroid bombardment contributed to extensive melting and resurfacing of ancient (>3.5 Ga) Mars. Evidence from the largest impact structures on Earth and the Moon suggests that vertically thick impact melt sheets experience chemical differentiation. Recently observed materials in Gale crater are enriched in alkalis (up to 14 wt% Na2O + K2O) and silica (up to 67 wt% SiO2) with low MgO (<5 wt%). This differentiation trend has previously been attributed to fractional crystallization of mantle-derived magmas. The influence of impact melting on Mars' crustal petrology, however, has not been explored in depth. In this study, we investigate differentiation of impact-generated melts as a mechanism for petrogenesis of evolved rocks on Mars. We scale melt volumes for multiple impact magnitudes and employ the MELTS algorithm to model mineral phase equilibria in those melts as they crystallize. We consider a range of possible melt geometries, oxidation states (IW to QFM), crystallization regimes (equilibrium and fractional), and water contents (anhydrous to water-saturated). Moderate pressure and dissolved volatiles are required to produce alkaline differentiation trends in mafic melts. Large impacts that melt a water-bearing early basaltic crust provide a mechanism to generate wet magmas on Mars, consistent with some modeled Martian differentiation trends. Low-degree partial melting and/or an alkali-enriched source are needed to generate observed highly alkaline compositions, motivating future work to further assess whether these conditions can be achieved in impact-generated melts.

Granites of the intracontinental termination of a magmatic arc: an example from the Ediacaran Araçuaí orogen, southeastern Brazil

The Araçuaí orogen of southeastern Brazil together with the West Congo belt of central West Africa form the Araçuaí–West Congo orogen generated during closure of a terminal segment of the Neoproterozoic Adamastor Ocean. Corresponding to an embayment in the São Francisco–Congo Craton, this portion of the Adamastor was only partially floored by oceanic crust. The convergence of its margins led to the development of the Rio Doce magmatic arc between 630Ma and 580Ma. The Rio Doce magmatic arc terminates in the northern portion of the Araçuaí orogen. Granitic plutons exposed in the northern extremity of the arc provide a rare opportunity to study magmatism at arc terminations, and to understand the interplay between calc-alkaline magma production and crustal recycling. The plutons forming the terminus of the arc consist of granodiorites, tonalites and monzogranites similar to a magnesian, slightly peraluminous, calcic- (68%) to calc-alkaline (24%), with minor alkali-calcic (8%) facies, medium- to high-K magmatic series. Although marked by negative Nb–Ta, Sr and Ti anomalies, typically associated with subduction-related magmas, the combined Sr, Nd and Hf isotopic data characterize a crustal signature related to anatexis of metamorphosed igneous and sedimentary rocks, rather than fractional crystallization of mantle-derived magmas. Zircon U–Pb ages characterizes two groups of granitoids. The older group, crystallized between 630 and 590Ma, experienced a migmatization event at ca. 585Ma. The younger granitoids, emplaced between 570 and 590Ma, do not show any evidence for migmatization. Most of the investigated samples show good correlation with the experimental compositional field of amphibolite dehydration-melting, with some samples plotting into the field of greywacke dehydration-melting. The studied rocks are not typical I-type or S-type granites, being particularly similar to transitional I/S-type granitoids described in the Ordovician Famatinian arc (NW Argentina). We suggest a hybrid model involving dehydration-melting of meta-igneous (amphibolites) and metasedimentary (greywackes) rocks for magma production in the northern termination of the Rio Doce arc. The real contribution of each end-member is, however, a challenging work still to be done.

Petrological and Geochemical Characteristics of Miocene-Pleistocene Basaltic Alkaline Subvolcanics of Gharyan District, NW Libya

Gharyan basaltic subvolcanic bodies, in the north-westernmost Libya, are characterized by extreme irregularity, having short length, numerous braches and abruptly change in thickness with a wide GPS coordinates. Commonly occurring as dykes, a few of them can be classified as small stocks, and others are sills. They have strike N 70-80˚ W. They are nepheline-normative alkaline basalts which are comprised of four distinct suites, namely, olivine basalt, basalt, trachy-basalt and trachy-andesite. The trachy- basalt and trachy-andesite are the end differentiation products of alkali-basalt series. The magmatism that gave rise the Miocene- Pleistocene basaltic dykes of Gharyan district represent intraplate alkaline volcanics generated by a young mantle plume but was contaminated by little crustal materials or were products of fractional crystallization of mantle-derived magma. Such magma is thought to be similar to the ocean island basalt formed in within- continental plate setting. Based on the field observation and K-Ar dating (6) subdivided the volcanic activity along eastern scarp of Jabal Nafusah (Gharyan district) into an old lava series, young lava series, volcanic cones, phonolite dome-shaped intrusions (47.0±1.2-37.7 Ma) and pyroclastic rocks. The young lava series (6.0±1.3-2.3±0.3 Ma) is mostly less common and restricted to the periphery of the old lava series or sometimes occupies ancient burial wadis. Late volcanic centers (7.7±1.5-1.3±0.5 Ma) usually form very small eruptive products which erupted from central volcanoes and dykes. Buserwil and Wadsworth (6) reported that both old and young lava series are composed of basaltic andesites. The late volcanic centers are generally basanitic suite with minor related differentiates, while the domes are all phonolitic suite. The present study intends to outline and compare the petrological and geochemical characteristics of the sheet-subvolcanic intrusions (dykes and sills), and briefly discusses some petrogenetic implications of the results obtained.

The Aguilar pluton (23°12′ S–65°40′ W; NW Argentina): Petrological implications on the origin of the Late Jurassic intraplate magmatism in the Central Andes

The Late Jurassic Aguilar pluton is located in NW Argentina, about 300–400 Km east of the Tarapacá basin, representing the backarc basin linked to the Jurassic volcanic arc. This small-size and compositionally heterogeneous pluton intruded the metasedimentary rocks of the Ordovician Santa Victoria Group, along the Cobres-Salinas Grandes lineament. A revision of published geochemical data in the light of new field and petrological results, allows us to propose a model concerning the petrogenesis and emplacement mechanisms of Aguilar pluton and to discuss its geodynamic setting. The pluton is mainly composed of metaluminous and nearly peraluminous granitoids, showing the geochemical characteristics of ferroan granites. The volumetrically subordinate mafic rocks are both ne- and hy-normative, and their primary magmas were generated by partial melting of a pristine Proterozoic mantle. Aguilar rocks display a rather limited range in (87Sr/86Sr)i, compared to the entire rift-related plutonic suite, i.e., 0.703198–0.704601, and εNdt from −1.06 to 3.82, calculated at 149 Ma. Fractional crystallization of mantle-derived magmas and crustal contamination processes explain the evolution to produce strongly silica-oversaturated magmas, which emplaced in the continental crust. The petrological data indicate that magma emplacement and cooling occurred at rather shallow depth. Multiple injections of magma batches into the magma chamber caused mingling and mixing processes early in the crystallization history. The Aguilar pluton is one of the several igneous complexes whose formation was associated with the extensional tectonics active during Late Jurassic-Early Cretaceous in NW Argentina. Based on the geological position and the igneous rocks affinity, we exclude that the Late Jurassic magmatism was generated in an orogenic setting and envisage that it was linked to the early extensional phase that preceded the Cretaceous continental rifting, related to the break–up of the South America-Africa continents.

Source of Mesozoic intermediate-felsic igneous rocks in the North China craton: Granulite xenolith evidence

Four intermediate to felsic igneous rocks from the Zhangjiakou region, along the northern margin of the North China craton, have magmatic zircon U–Pb ages from 122 to 144Ma. Two of these samples have inherited zircon U–Pb ages of ~2.5Ga, similar to the zircon ages of rocks from the surrounding granulite terrain. Zircons from two intermediate composition granulite xenoliths (JN0811 and JN0919) in the nearby Cenozoic Hannuoba basalts yield two groups of ages. The rims have concordant Mesozoic ages mostly between 120 and 145Ma, coeval with the Mesozoic intermediate-felsic magmatism in the region, while the cores have discordant U–Pb ages with upper-intercepts of ~2.5Ga, overlapping the zircon ages of granulite terrain rocks, and lower-intercept ages of ~130Ma, approximating the ages of the Mesozoic intermediate-felsic magmatism. The Sr–Nd isotopic compositions of the Mesozoic intermediate-felsic igneous rocks are completely different from those expected for basaltic melts from either the lithospheric mantle or the asthenospheric mantle, precluding a derivation by extensive fractional crystallization of mantle-derived magmas. The lack of correlation between (86Sr/87Sr)i, εNd(t) and SiO2 for the Mesozoic igneous rocks, the very narrow range of zircon εHf(t) for individual intermediate-felsic igneous rocks, and simple binary mixing calculations argue against them being formed by mixing between mantle-derived magma and preexisting crust that has extremely evolved Sr–Nd isotopic compositions like granulite xenoliths JN0811 and JN0919. Hf isotopic compositions of the Mesozoic zircons and whole-rock geochemistry show that the granulite xenoliths with extremely evolved Sr–Nd isotopic compositions have not undergone partial melting during the Mesozoic and thus do not contribute to the Mesozoic intermediate-felsic magmas. Further comparisons show that the source rocks for the Mesozoic intermediate-felsic magmas likely were late Archean lower crustal rocks similar in chemical and isotopic compositions to rocks with SiO2<62wt.% from the granulite terrain. If so, pyroxene-rich mafic granulite xenoliths found in the Hannuoba basalts could be restites left behind after the partial melting of the late Archean lower crust. The results shed new light on the origin for Mesozoic zircons in granulite xenoliths from Nushan, at the southern margin of the North China craton. We suggest that the widespread Mesozoic intermediate-felsic igneous rocks in the eastern North China craton are most likely derived from partial melting of the intermediate-mafic rocks of the late Archean lower crust.

Paleozoic alkaline volcanism: geochemistry and petrogenesis of Um Khors and Um Shaghir trachytes of the central Eastern Desert, Egypt

The Um Khors and Um Shaghir trachyte (UKT and UST) plugs and sheets represent two conspicuous outcrops of Paleozoic alkaline volcanism in the central Eastern Desert of Egypt. The trachyte magmatisms erupted along the Pan-African NW-trending shear zone (302 ± 15 and 273 ± 15 Ma, respectively) and intruded the Late Proterozoic rocks of the studied area. The trachyte rocks consist mainly of sanidine, anorthoclase, albite, and quartz with a noticeable amount of aegirine–augite, aegirine, hedenbergite, and arvedsonite. The studied trachytes are moderately evolved in composition (with 62–67.5 wt.% SiO2) and exhibit a limited compositional range in most of the major elements. They are alkaline in nature and considered as silica-oversaturated rocks. The rare earth elements (REE) patterns are somewhat uniform and highly fractionated, being enriched in light REE over heavy REE and show prominent negative Eu anomalies. The UKT and UST are enriched in high field strength elements Nb, Zr, and Y, consistent with typical within-plate alkaline magmatisms of extensional tectonic regimes. They were generated through the fractional crystallization of mantle-derived magmas. Although the UST is younger than the UKT, they show approximately similar chemical compositional ranges of the most major and trace elements, with somewhat higher MgO, Cr, Ni, and Ba contents in the former. This may argue against the evolution of the UST via a continuous fractional crystallization of the residual magmatic melt of the UKT. Thus, the UKT and UST are genetically related but could be emplaced through two various magmatic pulses of the same parent source (i.e., asthenospheric mantle source) at different times. The ascending magmatisms were subjected to variable significant degrees of crustal contamination during their generation.

Fingerprinting sources of orogenic plutonic rocks from Variscan belt with lithium isotopes and possible link to subduction-related origin of some A-type granites

Lithium (Li) elemental and isotope data are presented for a suite of plutonic rocks (I-, S-, A- and S S-type, coexisting mafic bodies) from the Western Carpathians, Slovakia, that were generated throughout the complete Variscan orogenic cycle. I-type granites show a limited range of δ 7Li, contrasting with the large variations found in S-type granites and orthogneisses of distinct ages. This contradicts previous reports of predominantly light-Li S-type granites, sourcing metasedimentary lithologies, and isotopically heavier I-type granites, derived from meta-igneous sources. An almost exclusively heavy Li isotope signature (δ 7Li > 4.7‰) found in four out of five A-type granites rules out several commonly accepted petrogenetic scenarios such as remelting of granulitic residue after formation of I-type granitic melts, anatexis of calc-alkaline meta-igneous crust or extensive closed-system fractional crystallization of mantle-derived magmas. Instead, it most probably reflects a derivation of A-type granites from a mantle wedge modified by slab-derived fluids during an Early Variscan or Pan-African subduction episode. In contrast, the distinct and lower δ 7Li values (< 1.2‰) in the contemporaneous S S-type granites can be related to their likely pelitic parentage. Mafic rocks (gabbros and diorites), associated with several occurrences of granites, are uniformly Li-rich and isotopically light (<−0.5‰), precluding a direct derivation from the mantle. These signatures testify to their cumulate origin whereby kinetic effects may be a viable explanation for the light Li isotope compositions, associated with diffusive redistribution of Li between mantle-derived mafic melts and acid magmas. This corroborates recent studies on faster diffusion of 6Li compared with 7Li in natural systems. Taken together, the presumed dichotomy between the sources and processes leading to generation of S- and I-type granitic magmas does not seem to be reflected by Li isotope signatures in a simple and globally valid manner. Interpretation of Li isotope compositions thus needs to be paralleled by other available information on petrology, whole-rock geochemistry and the magmatic context.

A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China

Three groups of Mesozoic shoshonitic or high-K calc-alkaline intrusive rocks are identified in Dabieshan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic zone, east-central China and they are related to: (I) slab breakoff; (II) magmatic underplating; and (III) doming. Group-I, the slab breakoff-type, consists of late Triassic (∼210 Ma) mafic monzodiorites. It has moderate to high Sr, and low Rb and Ba abundances, and moderate light rare earth element (LREE)/heavy rare earth element (HREE) and K/Rb ratios. Group-II, the underplating-type, consists mainly of middle Jurassic–early Cretaceous (160–120 Ma) hornblende quartz monzonitic, biotite monzogranitic, and syenogranitic plutons, characterized by relatively high LREE/HREE and K/Rb ratios, and by a large range in concentration of Sr and Ba, coupled with much smaller range in Rb. Group-III, the doming-type, is represented by Cretaceous (125–95 Ma) granitic stocks and granitic porphyry. Compared with group-II, it has high Rb, Y and HREE abundances, low Sr and Ba abundances and low LREE/HREE and K/Rb ratios. All groups have similar Nd and Sr isotopic compositions. Among the three groups, post-collisional granitoid magmatism (group-II) with ages of 160 to 120 Ma, post-dating the HP and UHP metamorphism at 245 to 220 Ma, is the most abundant in the Dabieshan area. The post-collisional granitoid plutons were initially emplaced at different levels ranging from mid-crust to near-surface. This study shows that the whole-rock chemistry of the granitoids vary systematically with crystallization pressures. For example, K 2O, normative Or, Rb and Zr show the strongest increase with decreasing pressure, whereas Ba, Nb, Nd, Yb, MnO, and normative An decrease upward in the Dabie Block. It is suggested that ascent of differentiated, buoyant liquids, combined with fractionation paired with assimilation (AFC), is responsible for the vertical variation. Geological, geochemical and petrological data indicate that group-I could have been generated by partial melting of enriched subcontinental lithosphere mantle due to slab breakoff. Group-II rocks could have been produced mainly from crustal assimilation/melting and fractional crystallization of mantle-derived magmas, whereas group-III magma could have derived from anatexis of the Dabie complex and was highly evolved in a hot doming setting. The late Triassic-early Jurassic slab breakoff may be responsible for the exhumation of UHP rocks through the mantle. The voluminous granitic emplacement together with an episode of rapid denudation suggests that magmatic underplating and inflation could have played a role in the Middle Jurassic–Early Cretaceous rapid exhumation of Dabieshan.

Liquid immiscibility between nephelinite and carbonatite from 1.0 to 2.5 GPa compared with mantle melt compositions

In order to define the conditions for the formation of immiscible carbonatite magmas in the lithosphere and in the crust, we have conducted phase equilibrium experiments to determine the effect of pressure and temperature on the silicate-carbonate liquid miscibility gap in bulk compositions appropriate for magmas in the upper mantle. A primitive (magnesian) nephelinite (NEPH) was used as a starting material, mixed with carbonates. Experiments were made with mixtures in the joins NEPH-dolomite-Na2CO3 (NEPH-Dol-NC) at 1.0 to 2.5 GPa, and NEPH-calcite (NEPH-CC) at 1.0 GPa. The miscibility gap was intersected by the join NEPH-Dol-NC (liquids with olivine), but not by NEPH-CC. Together with previous results for the Mg-free system (Na2O-CaO-Al2O3-SiO2-CO2), it was established that the size of the miscibility gap for magnesian compositions increases with decreasing pressures from depths of ˜100 km to ˜ 35 km; it increases further as compositions are changed by decreasing Mg/Ca. The maximum CaCO3 in liquids associated with the miscibility gap is 50 wt % for Mg-bearing liquids, and 80 wt % for Mg-free liquids. There is no experimental evidence for nearly pure-CaCO3 immiscible liquids, but abundant evidence for the precipitation of rounded calcite crystals from carbonate-rich liquids. The join NEPH-CC locates a piercing point on the liquidus field boundary for coprecipitation of olivine and calcite at NEPH50CC50 (wt %), part of the silicate-carbonate liquidus field boundary which defines the locus of liquids formed from carbonate-peridotites. The miscibility gap results are compared with magmas formed during partial fusion of CO2-bearing mantle peridotites, and during fractional crystallization of mantle-derived magmas. None of the probable magma paths in mantle processes intersects the miscibility gap. CO2-bearing mantle-derived alkalic magmas such as nephelinites and melilitites may fractionate during uprise through the mantle and crystallization within the crust. The compositions of these evolved nephelinites and phonolites approach the silicate side of the miscibility gap, confirming the probable generation of immiscible, alkalic carbonate-rich liquids at crustal pressures.

Non-monotonic chemical and O, Sr, Nd, and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff, Colorado

Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO2, fiamme group 7) had δ18O= +8.5, initial 87Sr/86Sr=0.7099, eNd, and 206Pb/204Pb=17.80, suggesting that the magma was produced by ∼50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO2, fiamme group 1) depart from the mafic “base-level” composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. δ18O values decrease by as much as 1.5‰ from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. δ18O values abruptly inerease by up to 1.5‰ between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial 87Sr/86Sr ratios in phenocrysts are as high as 0.7170. e values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. 206Pb/204Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.