scholarly journals Mesoscopic and Microscopic Magmatic Structures in the Quxu Batholith of the Gangdese Belt, Southern Tibet: Implications for Multiple Hybridization Processes

2021 ◽  
Vol 9 ◽  
Author(s):  
Xuxuan Ma ◽  
Zhongbao Zhao ◽  
Wenrong Cao ◽  
He Huang ◽  
Fahui Xiong ◽  
...  
Keyword(s):  
Magma Mixing ◽  
Mafic Magma ◽  
Intermediate Stage ◽  
Primitive Mantle ◽  
Chemical Compositions ◽  
Southern Tibet ◽  
Hybrid Products ◽  
Microgranular Enclaves ◽  
Juvenile Crust ◽  
Excellent Source

The Quxu batholith of the Gangdese magmatic belt, southern Tibet, comprises predominantly Early Eocene calc-alkaline granitoids that feature a variety of types of magmatic microgranular enclaves and dikes. Previous studies have demonstrated that magma mixing played a crucial role in the formation of the Quxu batholith. However, the specific processes responsible for this mixing/hybridization have not been identified. The magmatic microgranular enclaves and dikes preserve a record of this magma mixing, and are therefore an excellent source of information about the processes involved. In this study, mesoscopic and microscopic magmatic structures have been investigated, in combination with analyses of mineral textures and chemical compositions. Texturally, most of the enclaves are microporphyritic, with large crystals such as clinopyroxene, hornblende, and plagioclase in a groundmass of hornblende, plagioclase, and biotite. Two types of enclave swarms can be distinguished: polygenic and monogenic swarms. Composite dikes are observed, and represent an intermediate stage between undisturbed mafic dike and dike-like monogenic enclave swarms. Our results reveal three distinct stages of magma mixing in the Quxu batholith, occurring at depth, during ascent and emplacement, and after emplacement, respectively. At depth, thorough and/or partial mixing occurred between mantle-derived mafic and crust-derived felsic magmas to produce hybrid magma. The mafic magma was generated from the primitive mantle, whereas the felsic end-member was produced by partial melting of the preexisting juvenile crust. Many types of enclaves and host granitoids are thus cogenetic, because all are hybrid products produced by the mixing of the two contrasting magmas in different proportions. In the second stage, segregation and differentiation of the hybrid magma led to the formation of the host granitoids as well as various types of magmatic microgranular enclaves. At this stage, mingling and/or local mixing happened during ascent and emplacement. In the final stage, mafic or hybrid magma was injected into early fractures in the crystallizing and cooling pluton to form dikes. Some dikes remained undisturbed, whereas others experienced local mingling and mixing to form composite dikes and eventually disturbed dike-like monogenic enclave swarms. In summary, our study demonstrates the coupling between magmatic texture and composition in an open-system batholith and highlights the potential of magmatic structures for understanding the magma mixing process.

scholarly journals The Role of Magma Mixing in Generating Granodioritic Intrusions Related to Cu–W Mineralization: A Case Study from Qiaomaishan Deposit, Eastern China

Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 171 ◽  
Author(s):  
Huasheng Qi ◽  
Sanming Lu ◽  
Xiaoyong Yang ◽  
Jianghong Deng ◽  
Yuzhang Zhou ◽  
...  
Keyword(s):  
Magma Mixing ◽  
Eastern China ◽  
Mafic Magma ◽  
Enriched Mantle ◽  
Microgranular Enclaves ◽  
Intrusive Rocks ◽  
Granodiorite Porphyry ◽  
Shallow Crust ◽  
T Values ◽  
Host Rocks

The newly exploited Qiaomaishan Cu−W deposit, located in the Xuancheng ore district in the MLYRB, is a middle-sized Cu–W skarn-type polymetallic deposit. As Cu–W mineralization is a rare and uncommon type in the Middle-Lower Yangtze River Belt (MLYRB), few studies have been carried out, and the geochemical characteristics and petrogenesis of Qiaomaishan intrusive rocks related to Cu–W mineralization are not well documented. We studied two types of ore-bearing intrusive rocks in the Qiaomaishan region, i.e., pure granodiorite porphyry and granodiorite porphyry with mafic microgranular enclaves (MMEs). Age characterization using zircon LA–ICP–MS showed that they were formed almost simultaneously, around 134.9 to 135.1 Ma. Granodiorite porphyries are high Mg# adakites, characterized by high-K calc-alkaline and metaluminous features that are enriched in LILEs (e.g., Sr and Ba) and LREEs, but depleted in HFSEs (e.g., Nb, Ta, and Ti) and HREEs. Moreover, they have enriched Sr–Nd–Hf isotopic compositions (with whole-rock (87Sr/86Sr)i ratios (0.706666−0.706714), negative εNd(t) values of −9.1 to −8.6, negative zircon εHf(t) values of −12.2 to −6.7, and two-stage Hf model ages (TDM2) between 1.5 and 2.0 Ga). However, compared to host rocks, the granodiorite porphyry with MMEs shows variable geochemical compositions, e.g., high Mg#, Cr, Ni, and V contents and enriched with LILEs. In addition, they have more depleted ISr, εNd(t), and εHf(t) values (0.706025 to 0.706269, −6.4 to −7.4, and −10.6 to −5.7, respectively), overlapping with regions of Early Cretaceous mafic rocks derived from enriched lithospheric mantle in the MLYRB. Coupled with significant disequilibrium textures and geochemical features of host rocks and MMEs, we propose that those rocks have resulted from mixing the felsic lower crust-derived magma and the mafic magma generated from the enriched mantle. The mixed magmas subsequently rose to shallow crust to form the ore-bearing rocks and facilitate Cu–W mineralization.

scholarly journals GEOCHEMICAL CHARACTERISTICS OF THE ERENLERDAGI VOLCANICS, KONYA, CENTRAL TURKEY

2017 ◽  
Vol 50 (4) ◽  
pp. 2057
Author(s):  
C. Uyanık ◽  
K. Koçak
Keyword(s):  
Fractional Crystallization ◽  
Magma Mixing ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Calc Alkaline ◽  
High K ◽  
Microgranular Enclaves ◽  
Mafic Microgranular Enclaves ◽  
Central Turkey ◽  
Pyroclastic Fall

Late Miocene to Pliocene volcanism produced lava domes with mafic microgranular enclaves (MMEs), nuée ardentes and pyroclastic fall and flow (ignimbrites) deposits in the WSW and NW of Konya city. All samples are predominantly high K-calc alkaline in composition but calc-alkaline and shoshonitic composition also exist. The felsic volcanics are mainly dacite, andesite, basaltic trachyandesite and rare trachyandesite in compositon. But, the MMEs have basaltic andesite and andesite compositon. SiO2 increases with decreasing TiO2, FeOt, MgO and CaO, suggesting fractional crystallization of mafic minerals. All samples have fractionated chondritenormalised REE pattern (La/YbN: 6.7-18.1), and negative Eu anomaly (Eu/Eu*: 0.67- 0.89), indicating plagioclase fractionation. In primitive mantle-normalized spider diagram, the samples show an enrichment in large ion litophile elements (LILE) such as Cs and Ba, and depletion in high field strength elements (HFSE), e.g. Dy and Y. They show negative Nb, Ta and Ti anomalies, indicating a subduction signature for their genesis. Based on geochemical data, the volcanics are suggested to have been formed by Assimilation-Fractional Crystallization (AFC) and/or magma mixing process. Various geotectonic diagrams imply volcanic arc to post collisional setting for the samples.

Enclaves and their bearing on the origin of the Cornubian batholith, southwest England

1995 ◽  
Vol 59 (395) ◽  
pp. 273-296 ◽  
Author(s):  
James A. Stimac ◽  
Alan H. Clark ◽  
Yanshao Chen ◽  
Sammy Garcia
Keyword(s):  
Magma Mixing ◽  
Mafic Magma ◽  
Source Rocks ◽  
Coarse Grained ◽  
Contact Aureole ◽  
Igneous Origin ◽  
Microgranular Enclaves ◽  
Diverse Origin ◽  
Mineral Compositions ◽  
Thin Crust

AbstractEnclaves of diverse origin are present in minor amounts in the coarse-grained biotite granites of the Cornubian batholith, southwest England. The most common enclave type is layered, rich in biotite, cordierite and aluminosilicates, and has textures and compositions that reveal variable degrees of melt extraction from metasedimentary source rocks. Rare sillimanite-bearing enclaves represent residual material, either from the region of magma generation or its ascent path, but most such enclaves were probably derived from the contact aureole closer to the present level of exposure. These non-igneous enclaves (NIE) and their disaggregation products are present in all major plutons, comprising from < 2 to 5 vol.% of the granites. Enclaves of igneous origin are also present in all major plutons except Carnmenellis, generally comprising < 1 vol.% of the granites. The most common type is intermediate in composition, with microgranular texture, and mineral compositions and textures consistent with an origin by magma mixing. Large crystals of K-feldspar, plagioclase and quartz, common in these microgranular enclaves (ME) but absent in NIE, represent phenocrysts derived from the silicic end-member during magma mixing events rather than products of metasomatism as suggested previously. Although the composition of the mafic end-member (basaltic or lamprophyric) involved in the mixing process is poorly constrained, the presence of ME in the granites, and the preponderance of mantle-derived mafic rocks in the coeval Exeter Volcanics, indicate that mafic magma injection into the crust was a factor in the generation of the batholith. Advection of sub-crustal heat provides an explanation for large-volume crustal melting in regions of relatively thin crust such as southwest England.

A message from the depth: the origin of the amphibole crystal clots with pyroxene cores in the Late Pleistocene Ciomadul dacites, Romania

2021 ◽  
Author(s):  
Barbara Cserép ◽  
Zoltán Kovács ◽  
Kristóf Fehér ◽  
Szabolcs Harangi
Keyword(s):  
Inner Core ◽  
Magma Mixing ◽  
Outer Part ◽  
Mafic Magma ◽  
Outer Zone ◽  
Magma Reservoir ◽  
Explosive Eruptions ◽  
Innovation Fund ◽  
Amphibole Crystal ◽  
Crustal Magma

&lt;p&gt;Identification of trans-crustal magma reservoir processes beneath volcanoes is a crucial task to better understand the behaviour and possible future activities of volcanic systems. Detailed petrological investigations have a fundamental role in such studies. Dacitic magmas are usually formed in an upper crustal magma reservoir by complex open-system processes including crystal fractionation and magma mixing following recharge events. Conditions of such processes are usually constrained by crystal-scale studies, whereas there is much less information about the petrogenetic processes occurring in the lower crustal hot zone. Here we provide insight into such processes by new results on amphibole crystal clots found in dacitic pumices from an explosive volcanic suite of the Ciomadul volcano, the youngest one in eastern-central Europe.&lt;/p&gt;&lt;p&gt;Amphibole is a common mineral phase of the Ciomadul dacites, occuring as phenocrysts and antecrysts, but occasionally they also form crystal clots with an inner core of either pyroxene or olivine with high Mg-numbers. Olivine is observed mostly in the 160-130 ka lava dome rocks, whereas the younger explosive eruption products are characterised by orthopyroxene and clinopyroxene. Such mafic crystal clots are most common in the pumices of the earliest explosive eruptions, which occurred after long quiescence at 56-45 ka. The most common appearance has high-Mg pyroxene core (mg#: 0.76-0.92) rimmed by amphibole. Two types of amphibole are found in such clots: irregular zone of actinolite to magnesio-hornblende directly around orthopyroxene and high Mg-Al pargasitic amphibole as the outer zone. Several crystal clots contain smaller amphibole crystals with diffuse transition to clinopyroxene at the inner part and complexly zoned amphibole with biotite inclusions in the outer part. These amphibole and pyroxene have lower Mg-number (&lt; 0.80), and higher MnO content (up to 0.52 wt%) than the most common type. In both cases, amphibole could be a peritectic product of earlier-formed pyroxenes, which reacted with water-rich melt at higher and lower temperatures, respectively. Actinolite to magnesio-hornblende at the contact represents a transitional phase between pyroxene and the newly formed amphibole. In a few cases, crystal clots contain amphibole inclusions in pyroxene macrocrysts. These amphiboles have a particular composition not yet reproduced by experiments: they have high mg# (&gt;0.86), but low tetrahedral Al (0.9-1.0 apfu) and usually high Cr content (Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; is up to 0.9 wt%), similar to the orthopyroxene and clinopyroxene hosts (0.26-0.71 and 0.78-0.89 wt%, respectively). We interpret these amphiboles as an early formed liquidus phase crystallized along with pyroxene from an ultra-hydrous mafic magma. Occasionally, crystal clots are complexly zoned amphibole macrocrysts with dispersed clinopyroxene inclusions. The amphibole has a wide compositional range, usually with high Mg-Al pargasitic core. These amphiboles could have formed by peritectic reaction between clinopyroxene and a water-rich melt.&lt;/p&gt;&lt;p&gt;The observed mafic crystal clots in the dacites indicate the presence of strongly hydrous mafic magmas accumulated probably at the crust-mantle boundary. During mafic recharge, volatile transfer may contribute to the crystal mush rejuvenation at shallow depth and triggering explosive eruptions.&lt;/p&gt;&lt;p&gt;This research was financed by the Hungarian National Research, Development and Innovation Fund (NKFIH) within K135179 project.&lt;/p&gt;

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