scholarly journals Chemical Evolution of Calc-alkaline Magmas during the Ascent through Continental Crust: Constraints from Methana, Aegean Arc

2020 ◽  
Vol 61 (3) ◽  
Author(s):  
Milena V Schoenhofen ◽  
Karsten M Haase ◽  
Christoph Beier ◽  
Dominic Woelki ◽  
Marcel Regelous
Keyword(s):  
Trace Element ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Magma Mixing ◽  
Basaltic Magma ◽  
High Water ◽  
Calc Alkaline ◽  
Mafic Enclaves ◽  
Aegean Arc ◽  
Mineral Compositions

Abstract Quaternary calc-alkaline andesitic to dacitic lavas effusively erupted on top of about 30 km thick accreted continental crust at Methana peninsula in the western Aegean arc. We present new data of major and trace element concentrations as well as of Sr–Nd–Pb isotope ratios along with mineral compositions of Methana lavas and their mafic enclaves. The enclaves imply a parental basaltic magma and fractional crystallization processes with relatively little crustal assimilation in the deep part of the Methana magma system. The composition of amphibole in some mafic enclaves and lavas indicates deeper crystallization at ∼25 km depth close to the Moho compared with the evolved lavas that formed at <15 km depth. The presence of amphibole and low Ca contents in olivine suggest high water contents of ∼4 wt% in the primitive magmas at Methana. The compositions of andesitic and dacitic lavas reflect fractional crystallization, assimilation of sedimentary material, and magma mixing in the upper 15 km of the crust. The Methana magmas have fO2 of FMQ + 1 to FMQ + 2 (where FMQ is the fayalite–magnetite–quartz buffer) at temperatures of 1200 to 750 °C and the fO2 does not vary systematically from mafic to felsic compositions, suggesting that the mantle wedge was oxidized by sediment subduction. Amphibole is an important fractionating phase in the more evolved Methana magmas and causes significant changes in incompatible element ratios. Although xenocrysts and mineral compositions indicate magma mixing, the major and trace element variation implies only limited mixing between dacitic and basaltic melts.

scholarly journals Magma Mixing Genesis of the Mafic Enclaves in the Qingshanbao Complex of Longshou Mountain, China: Evidence from Petrology, Geochemistry, and Zircon Chronology

Minerals ◽  
2019 ◽  
Vol 9 (3) ◽  
pp. 195 ◽  
Author(s):  
Wenheng Liu ◽  
Xiaodong Liu ◽  
Jiayong Pan ◽  
Kaixing Wang ◽  
Gang Wang ◽  
...  
Keyword(s):  
Host Rock ◽  
Inductively Coupled Plasma ◽  
Magma Mixing ◽  
Coarse Grained ◽  
Calc Alkaline ◽  
Quartz Crystals ◽  
Alkaline Rocks ◽  
Mafic Enclaves ◽  
Normal Zoning ◽  
Host Rocks

The Qingshanbao complex, part of the uranium metallogenic belt of the Longshou-Qilian mountains, is located in the center of the Longshou Mountain next to the Jiling complex that hosts a number of U deposits. However, little research has been conducted in this area. In order to investigate the origin and formation of mafic enclaves observed in the Qingshanbao body and the implications for magmatic-tectonic dynamics, we systematically studied the mineralogy, petrography, and geochemistry of these enclaves. Our results showed that the enclaves contain plagioclase enwrapped by early dark minerals. These enclaves also showed round quartz crystals and acicular apatite in association with the plagioclase. Electron probe analyses showed that the plagioclase in the host rocks (such as K-feldspar granite, adamellite, granodiorite, etc.) show normal zoning, while the plagioclase in the mafic enclaves has a discontinuous rim composition and shows instances of reverse zoning. Major elemental geochemistry revealed that the mafic enclaves belong to the calc-alkaline rocks that are rich in titanium, iron, aluminum, and depleted in silica, while the host rocks are calc-alkaline to alkaline rocks with enrichment in silica. On Harker diagrams, SiO2 contents are negatively correlated with all major oxides but K2O. Both the mafic enclaves and host rock are rich in large ion lithophile elements such as Rb and K, as well as elements such as La, Nd, and Sm, and relatively poor in high field strength elements such as Nb, Ta, P, Ti, and U. Element ratios of Nb/La, Rb/Sr, and Nb/Ta indicate that the mafic enclaves were formed by the mixing of mafic and felsic magma. In terms of rare earth elements, both the mafic enclaves and the host rock show right-inclined trends with similar weak to medium degrees of negative Eu anomaly and with no obvious Ce anomaly. Zircon LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry) U-Pb concordant ages of the mafic enclaves and host rock were determined to be 431.8 5.2 Ma (MSWD (mean standard weighted deviation)= 1.5, n = 14) and 432.8 4.2 Ma (MSWD = 1.7, n = 16), respectively, consistent with that for the zircon U-Pb ages of the granite and medium-coarse grained K-feldspar granites of the Qingshanbao complex. The estimated ages coincide with the timing of the late Caledonian collision of the Alashan Block. This comprehensive analysis allowed us to conclude that the mafic enclaves in the Qingshanbao complex were formed by the mixing of crust-mantle magma with mantle-derived magma due to underplating, which caused partial melting of the ancient basement crust during the collisional orogenesis between the Alashan Block and Qilian rock mass in the early Silurian Period.

Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks

1984 ◽  
Vol 310 (1514) ◽  
pp. 675-692 ◽  
Keyword(s):  
Subduction Zone ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Magma Mixing ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Oceanic Lithosphere ◽  
Alkaline Basalt ◽  
Complex Process ◽  
Heterogeneous Mantle

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.

scholarly journals Mineral Compositions of Syn-collisional Granitoids and their Implications for the Formation of Juvenile Continental Crust and Adakitic Magmatism

2020 ◽  
Vol 61 (3) ◽  
Author(s):  
Yuanyuan Xiao ◽  
Shuo Chen ◽  
Yaoling Niu ◽  
Xiaohong Wang ◽  
Qiqi Xue ◽  
...  
Keyword(s):  
Trace Element ◽  
Partial Melting ◽  
Continental Crust ◽  
Parental Magma ◽  
Age Dating ◽  
Trace Element Geochemistry ◽  
Crustal Growth ◽  
Tio2 Content ◽  
Mineral Phases ◽  
Mineral Compositions

Abstract Continentalcollision zones have been proposed as primary sites of net continental crustal growth. Therefore, studies on syn-collisional granitoids with mafic magmatic enclaves (MMEs) are essential for testing this hypothesis. The Baojishan (BJS) and Qumushan (QMS) syn-collisional plutons in the North Qilian Orogen (NQO) on the northern margin of the Tibetan Plateau have abundant MMEs in sharp contact with host granitoids, sharing similar constituent minerals but with higher modal abundances of mafic minerals in MMEs. The QMS host granitoids have high Sr/Y and La/Yb ratios, showing adakitic compositions, which are differentfrom the BJS granitoids. Based on bulk-rock compositions and zircon U-Pb age-dating, recent studies on these two plutons proposed that MMEs represent cumulates crystallized early from the same magmatic system as their host granitoids, and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies. Here, we focus on the trace-element geochemistry of the constituent mineral phases of both MMEs and their host granitoids of the QMS and BJS plutons. Weshow that different mineral phases preferentially host different trace elements; for example, most rare earth elements (REEs and Y) reside in titanite (only found in the QMS pluton), amphibole, apatite, epidote and zircon (mostly heavy-REEs); and high-field-strength elements (HFSEs) reside in biotite, titanite, amphibole and zircon. Based on the mineral chemical data, we show that for these two plutons, MMEs are of similar cumulate origin, crystallized from primitive andesitic melts in the early stage of granitoid magmatism. The primitive andesitic melts for these syn-collisional granitoids are most likely produced by the partial melting of the oceanic crust, supporting the hypothesis of continental crustal growth considering the syn-collisional granitoids represent juvenile continental crust. As evidenced by distinct mineral compositions, the two plutons have different parental magma compositions, for example higher TiO2 content and higher Sr/Y and La/Yb ratios in the QMS parental magmas, a signature best understood as being inherited from the source. The higher TiO2 content of the parental magma for the QMS pluton leads to the common presence of titanite in the QMS pluton (absent in the BJS pluton), crystallization of which in turn controls the trace-element (REE, Y, Nb, Ta and others) systematics in the residual melts towards an adakitic signature. Therefore, parental magmas with high TiO2 content and high Sr/Y and La/Yb ratios, as well as their further fractionation of titanite, are important factors in the development of adakitic compositions, as represented by the QMS host granitoids. This model offers a new perspective on the petrogenesis of adakitic rocks. The present study further demonstrates that, in general, mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks.

scholarly journals Heat- and melt-fluxed melting of lower continental crust: Insights from two types of subduction-related granitoids in northeastern China and the implications for crustal reworking and growth

Lithosphere ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 488-506
Author(s):  
Xing-Hua Ma ◽  
Shi-Lei Qiao ◽  
Peng Xiang ◽  
Andrei V. Grebennikov ◽  
Renjie Zhou
Keyword(s):  
Continental Crust ◽  
Continental Margin ◽  
Lower Crust ◽  
Magma Mixing ◽  
Hybrid Origin ◽  
Plate Boundaries ◽  
Mafic Enclaves ◽  
Primary Location ◽  
Crustal Reworking ◽  
Zircon Saturation

AbstractConvergent plate boundaries are the primary location for the formation of continental crust by the intrusion of arc batholiths that contain essentially mantle-derived magmas. This paper presents two types of arc granitoids (enclave-free monzogranites and enclave-bearing granodiorites) in northeastern (NE) China to understand crustal evolution and growth in the eastern Asian continental margin. The monzogranites (189 Ma) show characteristics typical of upper continental crust, with high SiO2 contents and enrichment of K, Rb, and Pb. These monzogranites have low ISr (87Sr/86Sr) ratios (0.70378–0.70413) and positive εNd (t) (+2.2 to +2.3) and εHf (t) (+7.3 to +10.2) values. These features, combined with high zircon saturation temperatures (TZr > 800 °C), suggest that the monzogranites were generated by the heat-fluxed melting of juvenile lower crust. In contrast, the granodiorites (171 Ma) contain abundant coeval mafic enclaves and show relatively low silica contents, low TZr (748–799 °C), and particularly wide variation in εHf (t) (−3.5 to +5.6), implying a hybrid origin involving both mantle- and crust-derived components. Isotopic modeling indicates that mantle material accounts for around 60%–70% of the hybrid magmas by volume. The granodiorites have adakite-like signatures (e.g., Sr/Y > 21 and [La/Yb]N > 15), which may have been primarily caused by a process of magma mixing and hornblende-dominated fractional fractionation, rather than through melting of a subducting slab or thickened lower crust. The two distinct granitoids (monzogranites and granodiorites) represent continental crustal reworking and growth, respectively, related to the subduction of the Paleo-Pacific Plate beneath the eastern Asian continental margin during the Jurassic.

Contribution a l'etude du volcanisme de l'arc des Nouvelles-Hebrides; caracterisation de deux series magmatiques de l'ile d'Erromango

1979 ◽  
Vol S7-XXI (5) ◽  
pp. 631-641 ◽  
Author(s):  
G. Marcelot ◽  
C. Lefevre ◽  
P. Maillet ◽  
R. C. Maury
Keyword(s):  
Fractional Crystallization ◽  
Basaltic Magma ◽  
Island Arc ◽  
Calc Alkaline ◽  
Early Stages ◽  
New Hebrides ◽  
Volcanic Series

Abstract The volcanic series of Mt Rantop and Robertson's Thumb, Erromango Island, New Hebrides, formed by fractional crystallization of orogenic basaltic magma of near-island-arc tholeiitic type. Differentiation was controlled mainly by separation of plagioclase, olivine and clinopyroxene. The Mt Rantop series is predominantly tholeiitic (plagioclase at the liquidus, late appearance of magnetite, pigeonite in microphenocrysts, and Fe and Ti remaining constant or increasing in the early stages of differentiation); those of Robertson's Thumb are mostly calc-alkaline (magnetite at the liquidus, late appearance of plagioclase, olivine quickly becoming unstable, orthopyroxene in phenocrysts and early decrease of Fe and Ti). The compositional differences reflect higher fO <sub>2</sub> and PH <sub>2</sub> O in Robertson's Thumb during fractional crystallization.

Development of the Early Continental Crust. Part 1. Use of Trace Element Distribution Coefficient Models for the Protoarchean Crust

1972 ◽  
Vol 9 (12) ◽  
pp. 1577-1595 ◽  
Author(s):  
Denis M. Shaw
Keyword(s):  
Distribution Coefficient ◽  
Trace Element ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Distribution Coefficients ◽  
Element Distribution ◽  
Trace Element Distribution ◽  
Measured Distribution

The original continental crust developed as the residue from fractional crystallization of the mantle–crust system. Using measured distribution coefficients for K, Rb, Ba, Sr, La, Ce, Eu, Yb, and Ni, several crystallization models are tested for conformity with regional geochemical estimates of continental crustal composition.In spite of the uncertainties and approximations the predicted concentrations agree reasonably well with observation, except in the case of Yb.

The Petrology of the Tarosero Volcanic Complex: Constraints on the formation of extrusive agpaitic rocks

2021 ◽  
Author(s):  
S Braunger ◽  
M A W Marks ◽  
T Wenzel ◽  
A N Zaitsev ◽  
G Markl
Keyword(s):  
Trace Element ◽  
Water Activity ◽  
Fractional Crystallization ◽  
Genetic Relationships ◽  
Alkali Feldspar ◽  
High Water ◽  
Low Pressure ◽  
Water Soluble ◽  
Volcanic Complex ◽  
Rock Types

Abstract The Quaternary Tarosero volcano is situated in the East African Rift of northern Tanzania and mainly consists of trachyte lavas and some trachytic tuffs. In addition, there are minor occurrences of extrusive basalts, andesites, latites, as well as peralkaline trachytes, olivine trachytes and phonolites. Some of the peralkaline phonolites contain interstitial eudialyte, making Tarosero one of the few known occurrences for extrusive agpaitic rocks. This study investigates the genetic relationships between the various rock types and focuses on the peculiar formation conditions of the extrusive agpaitic rocks using a combination of whole-rock geochemistry, mineral chemistry, petrography, thermodynamic calculations, as well as major and trace element modelling. The Tarosero rocks formed at redox conditions around or below the fayalite-magnetite-quartz buffer (FMQ). During multi-level magmatic fractionation at depths between ∼40 km and the shallow crust, temperature decreased from > 1100 °C at near-liquidus conditions in the basalts to ∼ 700 °C in the peralkaline residue. Fractional crystallization models and trace element characteristics do not indicate a simple genetic relationship between the trachytes and the other rock types at Tarosero. However, the genetic relationships between the primitive basalts and the intermediate latites can be explained by high pressure fractional crystallization of olivine + clinopyroxene + magnetite + plagioclase + apatite. Further fractionation of these mineral phases in addition to amphibole and minor ilmenite led to the evolution towards the peralkaline trachytes and phonolites. The eudialyte-bearing varieties of the peralkaline phonolites required additional low-pressure fractionation of alkali feldspar and minor magnetite, amphibole and apatite. In contrast to the peralkaline trachytes and phonolites, the peralkaline olivine trachytes contain olivine instead of amphibole, thus indicating a magma evolution at even lower pressure conditions. They can be modelled as a derivation from the latites by fractional crystallization of plagioclase, clinopyroxene, magnetite and olivine. In general, agpaitic magmas evolve under closed system conditions which impedes the escape of volatile phases. In case of the extrusive agpaitic rocks at Tarosero, the early exsolution of fluids and halogens was prevented by a low water activity. This resulted in high concentrations of Rare Earth Elements (REE) and other High Field Strength Elements (HFSE) and the formation of eudialyte in the most evolved peralkaline phonolites. Within the peralkaline rock suite, the peralkaline olivine trachytes contain the lowest HFSE and REE concentrations, consistent with mineralogical evidence for a formation at a relatively high water activity. The lack of amphibole fractionation, which can act as a water buffer of the melt, as well as the evolution at relatively low pressure conditions caused the early exsolution of fluids and loss of water-soluble elements. This prevented a strong enrichment of HFSE and REE before the magma finally extruded.

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