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

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.

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Geochemistry of two metavolcanic arc suites from the Central Metasedimentary Belt of the Grenville Province, southeastern Ontario, Canada

Canadian Journal of Earth Sciences ◽

10.1139/e91-126 ◽

1991 ◽

Vol 28 (9) ◽

pp. 1429-1443 ◽

Cited By ~ 12

Author(s):

Luc Harnois ◽

John M. Moore

Keyword(s):

Rare Earth ◽

Rare Earth Elements ◽

Trace Element ◽

Partial Melting ◽

Volcanic Rocks ◽

Parental Magma ◽

Major Elements ◽

Grenville Province ◽

Element Abundances ◽

Felsic Rocks

Samples of two subalkaline metavolcanic suites, the Tudor formation (ca. 1.28 Ga) and the overlying Kashwakamak formation, have been analysed for major elements and 27 trace elements (including rare-earth elements). The Tudor formation is tholeiitic and contains mainly basaltic flows, whereas the Kashwakamak formation is calc-alkaline and contains mainly andesitic rocks with minor felsic rocks. The succession has been regionally metamorphosed to upper greenschist – lower amphibolite facies. Trace-element abundances and ratios indicate that rocks of the Tudor and Kashwakamak formations are island-arc type. Geochemical modelling using rare-earth elements, Zr, Ti, and Y indicates that the Tudor volcanic rocks are not derived from a single parental magma through simple fractional crystallization. Equilibrium partial melting of a heterogeneous Proterozoic upper mantle can explain the trace-element abundances and ratios of Tudor formation volcanic rocks. The intermediate to felsic rocks of the Kashwakamak formation appear to have been derived from a separate partial melting event. The data are consistent with an origin of the arc either on oceanic crust or on thinned continental crust, and with accretion of the arc to a continental margin between the time of extrusion of Tudor volcanic rocks and that of Kashwakamak volcanic rocks.

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Crustal assimilation in the Burnt Lake metavolcanics, Grenville Province, southeastern Ontario, and its tectonic significance

Canadian Journal of Earth Sciences ◽

10.1139/e17-101 ◽

1997 ◽

Vol 34 (9) ◽

pp. 1272-1285 ◽

Cited By ~ 5

Author(s):

T. E. Smith ◽

P. E. Holm ◽

N. M. Dennison ◽

M. J. Harris

Keyword(s):

Rare Earth ◽

Trace Element ◽

Partial Melting ◽

Continental Crust ◽

Fractional Crystallization ◽

Volcanic Rocks ◽

Lake Area ◽

Depleted Mantle ◽

Mafic Magmas ◽

Back Arc

Three intimately interbedded suites of volcanic rocks are identified geochemically in the Burnt Lake area of the Belmont Domain in the Central Metasedimentary Belt, and their petrogenesis is evaluated. The Burnt Lake back-arc tholeiitic suite comprises basalts similar in trace element signature to tholeiitic basalts emplaced in back-arc basins formed in continental crust. The Burnt Lake continental tholeiitic suite comprises basalts and andésites similar in trace element composition to continental tholeiitic sequences. The Burnt Lake felsic pyroclastic suite comprises rhyolitic pyroclastics having major and trace element compositions that suggest that they were derived from crustal melts. Rare earth element models suggest that the Burnt Lake back-arc tholeiitic rocks were formed by fractional crystallization of mafic magmas derived by approximately 5% partial melting of an amphibole-bearing depleted mantle, enriched in light rare earth elements by a subduction component. The modelling also suggests that the Burnt Lake continental tholeiitic rocks were formed by contamination – fractional crystallization of mixtures of mafic magmas, derived by ~3% partial melting of the subduction-modified source, and rhyolitic crustal melts. These models are consistent with the suggestion that the Belmont Domain of the Central Metasedimentary Belt formed as a back-arc basin by attenuation of preexisting continental crust above a westerly dipping subduction zone.

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U–Pb dates and trace-element geochemistry of zircon from migmatite, Western Gneiss Region, Norway: Significance for history of partial melting in continental subduction

Lithos ◽

10.1016/j.lithos.2013.02.003 ◽

2013 ◽

Vol 170-171 ◽

pp. 35-53 ◽

Cited By ~ 51

Author(s):

Stacia M. Gordon ◽

Donna L. Whitney ◽

Christian Teyssier ◽

Haakon Fossen

Keyword(s):

Trace Element ◽

Partial Melting ◽

Trace Element Geochemistry ◽

Element Geochemistry ◽

Western Gneiss Region ◽

Continental Subduction ◽

History Of

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

Journal of Petrology ◽

10.1093/petrology/egaa036 ◽

2020 ◽

Vol 61 (3) ◽

Cited By ~ 2

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.

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GEOCHEMISTRY AND TECTONIC SETTING OF ECLOGITE PROTOLITHS FROM KECHROS COMPLEX IN EAST RHODOPE (N.E. GREECE)

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.11659 ◽

2017 ◽

Vol 43 (5) ◽

pp. 2522 ◽

Cited By ~ 1

Author(s):

I. Baziotis ◽

E. Mposkos

Keyword(s):

Trace Element ◽

Partial Melting ◽

Tectonic Setting ◽

Tio2 Content ◽

Geochemical Evidence ◽

Element Concentrations ◽

Tectonic Environment ◽

Lree Enrichment ◽

Ree Patterns ◽

Lree Depletion

Eclogites and partially amphibolitized eclogites from the metamorphic Kechros complex in East Rhodope are studied in order to provide the geodynamic framework for the origin of their protoliths. Geochemical evidence from whole rock major and trace element concentrations shows two distinct protolith groups. The low-Fe-Ti eclogites (Charakoma locality) have low-TiO2 content (<0.67 wt%), negative Nb anomalies, positive Sr anomalies, small negative Zr and Hf anomalies and variable enrichments in LILE (e.g. Rb and Ba). The REE patterns are characterized by strong LREE enrichment (LaN/YbN=5.45-5.81), HREE depletion (GdN/YbN=1.60-1.63) and HREE abundance within the rangeof 9-10 × chondrite. The high-Fe-Ti eclogites (Kovalo and Virsini locality) have variable Sr contents, small to moderate LILE enrichment, HREE`s similar to MORB values and absence of Nb anomalies. The REE patterns of the Kovalo and Virsini eclogites are characterized by LREE depletion and relative flat MREE HREE patterns at approximately 20-30 × chondrite concentrations. Our results suggest that the protoliths of the Low-Ti eclogites show a continental rifting tectonic environment. In contrast, the protoliths of the High-Ti eclogites indicate formation of their protoliths by partial melting in an extensional oceanic environment.

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