Mineral and rock compositions of mafic enclaves in the Morton Gneiss

1980 ◽  
pp. 95-104 ◽  
Author(s):  
BRUCE V. NIELSEN ◽  
PAUL W. WEIBLEN
Keyword(s):  
Mafic Enclaves

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.

Petrogenesis of early Eocene granites and associated mafic enclaves in the Gangdese batholith, Tibet: Implications for net crustal growth in collision zones

Lithos ◽  
2021 ◽  
Vol 394-395 ◽  
pp. 106170
Author(s):  
Ding-Jun Wen ◽  
Xiu-Mian Hu ◽  
Jian-Sheng Qiu ◽  
Jin-Hai Yu ◽  
Rui-Qiang Wang ◽  
...  
Keyword(s):  
Crustal Growth ◽  
Early Eocene ◽  
Mafic Enclaves

scholarly journals Geochemical and Sr–Nd isotopic features of the Zaro volcanic complex: insights on the magmatic processes triggering a small-scale prehistoric eruption at Ischia island (south Italy)

2020 ◽  
Vol 109 (8) ◽  
pp. 2829-2849
Author(s):  
C. Pelullo ◽  
G. Cirillo ◽  
R. S. Iovine ◽  
I. Arienzo ◽  
M. Aulinas ◽  
...  
Keyword(s):  
Risk Mitigation ◽  
Mafic Magma ◽  
Small Scale ◽  
Volcanic Complex ◽  
Magmatic System ◽  
Mafic Enclaves ◽  
South Italy ◽  
Ischia Island ◽  
Magmatic Processes ◽  
Trace Element Contents

Abstract The prehistoric (< 7 ka) Zaro eruption at Ischia island (Southern Italy) produced a lava complex overlaying a pyroclastic deposit. Although being of low energy, the Zaro eruption might have caused casualties among the neolithic population that inhabited that area of Ischia, and damages to their settlements. A similar eruption at Ischia with its present-day population would turn into a disaster. Therefore, understanding the magmatic processes that triggered the Zaro eruption would be important for volcanic hazard assessment and risk mitigation, so as to improve a knowledge that can be applied to other active volcanic areas worldwide. The main Zaro lava body is trachyte and hosts abundant mafic and felsic enclaves. Here all juvenile facies have been fully characterized from petrographic, geochemical and isotopic viewpoints. The whole dataset (major and trace element contents; Sr–Nd isotopic composition) leads to rule out a genetic link by fractional crystallization among the variable facies. Thus, we suggest that the Zaro mafic enclaves could represent a deep-origin mafic magma that mingled/mixed with the main trachytic one residing in the Ischia shallow magmatic system. The intrusion of such a mafic magma into a shallow reservoir filled by partly crystallized, evolved magma could have destabilized the magmatic system presumably acting as a rapid eruption trigger. The resulting processes of convection, mixing and rejuvenation have possibly played an important role in pre- and syn-eruptive phases also in several eruptions of different sizes in the Neapolitan area and elsewhere in the world.

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.

scholarly journals Generation of Porphyritic and Equigranular Mafic Enclaves During Magma Recharge Events at Unzen Volcano, Japan

2005 ◽  
Vol 47 (2) ◽  
pp. 301-328 ◽  
Author(s):  
BRANDON L. BROWNE ◽  
JOHN C. EICHELBERGER ◽  
LINA C. PATINO ◽  
THOMAS A. VOGEL ◽  
JONATHAN DEHN ◽  
...  
Keyword(s):  
Unzen Volcano ◽  
Mafic Enclaves ◽  
Magma Recharge
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