Details
| Originalsprache | Englisch |
|---|---|
| Seiten (von - bis) | 467-484 |
| Seitenumfang | 18 |
| Fachzeitschrift | Lithos |
| Jahrgang | 119 |
| Ausgabenummer | 3-4 |
| Frühes Online-Datum | 7 Aug. 2010 |
| Publikationsstatus | Veröffentlicht - Okt. 2010 |
Abstract
Field observation, petrography and geochemistry of mafic enclaves/dikes and their host felsic rocks from the Meichuan pluton are used to propose a geodynamic model for the southern Dabie orogen, central China. The similar Sr-Nd isotopic ratios [εNd(t)=-15, (87Sr/86Sr)i=0.70550-0.70597] and zircon Hf isotopic ratios [εHf(t)=-25 to -16] indicate that the two types of magmatic rocks were formed by coeval felsic and mafic magmas during the Early Cretaceous (132±2Ma, zircon U-Pb age). The adakitic signatures of both the felsic and mafic rocks, such as very high Sr (770-1400ppm), high Sr/Y ratios (40-130), low Y (3.5-21ppm) and HREE concentrations are supposed to be features of the primary magmas, indicating that both of them were generated by partial melting of basaltic protoliths at great depths (>15kbar). The distinctive major element compositions of the felsic and mafic primary magmas could be attributable to different melting temperatures and melting degrees. The mafic enclaves/dike have distinctively high concentrations of MgO (4.4-5.8wt.%), Cr (229-374ppm) and Ni (75-163ppm), indicating a melt-mantle interaction in which olivine is partly consumed while orthopyroxene and/or pyrope are formed under high-pressure. Modeling suggests that 14% of peridotite relative to melt could have been consumed to elevate the Mg# of melt to the observed values (55-60), and that transformation from orthopyroxene to pyrope in mantle peridotite could have decreased the Al2O3 content from 18-19wt.% in the initial melts to ~15wt.% in the resultant mafic melts. Moderate negative zircon εHf(t) and bulk εNd(t) values also suggest contributions from both enriched lithospheric mantle and ancient lower crust. The results can best be explained by assuming that a block of amphibolite-composition lower continental crust was delaminated into the lithospheric mantle, leading to the formation of mafic magmas. This delamination is attributed to lithospheric extension and asthenospheric upwelling along the Yangtze River fault zone in the late Mesozoic which could have heated up the lithospheric mantle underneath the neighboring southern Dabie orogen. The strong input of heat triggered the delamination of the overlying thickened lower crust into lithospheric mantle by weakening the uppermost mantle and resulted in melting reactions in the delaminated crustal block to produce hot (~1100°C) intermediate adakitic magmas (SiO2=55-60wt.%). The ascent of these hot magmas and their underplating below the lower crust induced the generation of high-silica melts (SiO2=~70wt.%) at lower temperature (~925°C or less). Hybridization processes between the two magma types occurred during further ascent and emplacement in the crust, which could have led to the high concentrations of MgO (2.2-3.5wt.%), Cr (56-226ppm) and Ni (33-99ppm) in the host felsic rocks.
ASJC Scopus Sachgebiete
- Erdkunde und Planetologie (insg.)
- Geologie
- Erdkunde und Planetologie (insg.)
- Geochemie und Petrologie
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in: Lithos, Jahrgang 119, Nr. 3-4, 10.2010, S. 467-484.
Publikation: Beitrag in Fachzeitschrift › Artikel › Forschung › Peer-Review
}
TY - JOUR
T1 - Origin of high-Mg adakitic magmatic enclaves from the Meichuan pluton, southern Dabie orogen (central China)
T2 - Implications for delamination of the lower continental crust and melt-mantle interaction
AU - Zhang, Chao
AU - Ma, Changqian
AU - Holtz, Francois
N1 - Copyright: Copyright 2011 Elsevier B.V., All rights reserved.
PY - 2010/10
Y1 - 2010/10
N2 - Field observation, petrography and geochemistry of mafic enclaves/dikes and their host felsic rocks from the Meichuan pluton are used to propose a geodynamic model for the southern Dabie orogen, central China. The similar Sr-Nd isotopic ratios [εNd(t)=-15, (87Sr/86Sr)i=0.70550-0.70597] and zircon Hf isotopic ratios [εHf(t)=-25 to -16] indicate that the two types of magmatic rocks were formed by coeval felsic and mafic magmas during the Early Cretaceous (132±2Ma, zircon U-Pb age). The adakitic signatures of both the felsic and mafic rocks, such as very high Sr (770-1400ppm), high Sr/Y ratios (40-130), low Y (3.5-21ppm) and HREE concentrations are supposed to be features of the primary magmas, indicating that both of them were generated by partial melting of basaltic protoliths at great depths (>15kbar). The distinctive major element compositions of the felsic and mafic primary magmas could be attributable to different melting temperatures and melting degrees. The mafic enclaves/dike have distinctively high concentrations of MgO (4.4-5.8wt.%), Cr (229-374ppm) and Ni (75-163ppm), indicating a melt-mantle interaction in which olivine is partly consumed while orthopyroxene and/or pyrope are formed under high-pressure. Modeling suggests that 14% of peridotite relative to melt could have been consumed to elevate the Mg# of melt to the observed values (55-60), and that transformation from orthopyroxene to pyrope in mantle peridotite could have decreased the Al2O3 content from 18-19wt.% in the initial melts to ~15wt.% in the resultant mafic melts. Moderate negative zircon εHf(t) and bulk εNd(t) values also suggest contributions from both enriched lithospheric mantle and ancient lower crust. The results can best be explained by assuming that a block of amphibolite-composition lower continental crust was delaminated into the lithospheric mantle, leading to the formation of mafic magmas. This delamination is attributed to lithospheric extension and asthenospheric upwelling along the Yangtze River fault zone in the late Mesozoic which could have heated up the lithospheric mantle underneath the neighboring southern Dabie orogen. The strong input of heat triggered the delamination of the overlying thickened lower crust into lithospheric mantle by weakening the uppermost mantle and resulted in melting reactions in the delaminated crustal block to produce hot (~1100°C) intermediate adakitic magmas (SiO2=55-60wt.%). The ascent of these hot magmas and their underplating below the lower crust induced the generation of high-silica melts (SiO2=~70wt.%) at lower temperature (~925°C or less). Hybridization processes between the two magma types occurred during further ascent and emplacement in the crust, which could have led to the high concentrations of MgO (2.2-3.5wt.%), Cr (56-226ppm) and Ni (33-99ppm) in the host felsic rocks.
AB - Field observation, petrography and geochemistry of mafic enclaves/dikes and their host felsic rocks from the Meichuan pluton are used to propose a geodynamic model for the southern Dabie orogen, central China. The similar Sr-Nd isotopic ratios [εNd(t)=-15, (87Sr/86Sr)i=0.70550-0.70597] and zircon Hf isotopic ratios [εHf(t)=-25 to -16] indicate that the two types of magmatic rocks were formed by coeval felsic and mafic magmas during the Early Cretaceous (132±2Ma, zircon U-Pb age). The adakitic signatures of both the felsic and mafic rocks, such as very high Sr (770-1400ppm), high Sr/Y ratios (40-130), low Y (3.5-21ppm) and HREE concentrations are supposed to be features of the primary magmas, indicating that both of them were generated by partial melting of basaltic protoliths at great depths (>15kbar). The distinctive major element compositions of the felsic and mafic primary magmas could be attributable to different melting temperatures and melting degrees. The mafic enclaves/dike have distinctively high concentrations of MgO (4.4-5.8wt.%), Cr (229-374ppm) and Ni (75-163ppm), indicating a melt-mantle interaction in which olivine is partly consumed while orthopyroxene and/or pyrope are formed under high-pressure. Modeling suggests that 14% of peridotite relative to melt could have been consumed to elevate the Mg# of melt to the observed values (55-60), and that transformation from orthopyroxene to pyrope in mantle peridotite could have decreased the Al2O3 content from 18-19wt.% in the initial melts to ~15wt.% in the resultant mafic melts. Moderate negative zircon εHf(t) and bulk εNd(t) values also suggest contributions from both enriched lithospheric mantle and ancient lower crust. The results can best be explained by assuming that a block of amphibolite-composition lower continental crust was delaminated into the lithospheric mantle, leading to the formation of mafic magmas. This delamination is attributed to lithospheric extension and asthenospheric upwelling along the Yangtze River fault zone in the late Mesozoic which could have heated up the lithospheric mantle underneath the neighboring southern Dabie orogen. The strong input of heat triggered the delamination of the overlying thickened lower crust into lithospheric mantle by weakening the uppermost mantle and resulted in melting reactions in the delaminated crustal block to produce hot (~1100°C) intermediate adakitic magmas (SiO2=55-60wt.%). The ascent of these hot magmas and their underplating below the lower crust induced the generation of high-silica melts (SiO2=~70wt.%) at lower temperature (~925°C or less). Hybridization processes between the two magma types occurred during further ascent and emplacement in the crust, which could have led to the high concentrations of MgO (2.2-3.5wt.%), Cr (56-226ppm) and Ni (33-99ppm) in the host felsic rocks.
KW - Dabie orogen
KW - Delamination
KW - Eastern China
KW - High-Mg adakite
KW - Mafic enclave
UR - http://www.scopus.com/inward/record.url?scp=77956903102&partnerID=8YFLogxK
U2 - 10.1016/j.lithos.2010.08.001
DO - 10.1016/j.lithos.2010.08.001
M3 - Article
AN - SCOPUS:77956903102
VL - 119
SP - 467
EP - 484
JO - Lithos
JF - Lithos
SN - 0024-4937
IS - 3-4
ER -