TY - JOUR
T1 - An amphibolitic source for “adakitic” I-type plutons in continental collision zones
AU - Li, Qi Wei
AU - Nebel, Oliver
AU - Zhao, Jun Hong
AU - Wang, Rui
AU - Jacobsen, Yona
AU - Richter, Marianne
AU - Wang, Qiang
AU - Cawood, Peter A.
N1 - Funding Information:
This work was supported by the National Natural Science Foundation of China (Nos. 42025203 , 42225204 , 41903012 ) and 111 Project ( B18048 ), the Australian Research Council (grant FT140101062 to ON and FL160100168 to PAC), the Ministerio de Ciencia e Innovación of Spain ( FCJ2021-047355-1 to MR) and the MOST Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences ( MSFGPMR2022-6 ). We are grateful to Zhiyong Yan for helpful discussions. We thank two anonymous reviewers for constructive comments and Rosemary Hickey-Vargas for suggestions and editorial handling.
Publisher Copyright:
© 2023 Elsevier B.V.
PY - 2023/10/1
Y1 - 2023/10/1
N2 - Rocks with “adakitic” affinities are widespread in continental collision zones, recording important information about crustal thickening. Residual garnet during partial melting of the lower crust is traditionally considered to be responsible for heavy rare earth element and Y depletion, which is a key characteristic feature of these rocks. However, magmatic garnet is rarely observed in such adakitic igneous rocks and associated protolithic eclogite is also rarely reported. Instead, amphibole is a proposed abundant phase in lower arc crust. Whilst both garnet- and amphibole-rich assemblages can act as potential sources for intracrustal intermediate-silicic melts, their distinctive fractionation of fSm/Nd and fLu/Hf should, with time, lead to different Nd-Hf isotopic compositions in high Sr/Y rocks. Here we report whole-rock radiogenic Sr-Nd-Hf and stable Fe isotope compositions of well-characterized Oligocene to Miocene (33-14 Ma), post-collisional high-K adakitic intrusions from the Gangdese belt, South Tibet. The plutons have a low alumina saturation index and depleted Dy/Yb, pointing to negligible amounts of sediment contribution. They are characterized by variable initial 87Sr/86Sr ratios (0.7048 to 0.7078), mostly negative εNd (−4.9 to +0.3), positive εHf values (+1.5 to +8.3), and variable but heavy δ57Fe values ranging from +0.04 ± 0.08‰ to +0.35 ± 0.08‰. Modeling of Fe isotopes and fractionation factors of fSm/Nd and fLu/Hf indicate an amphibolite source. A Nd-Hf isotope correlation that is quasi-parallel to the mantle array can be best explained by the evolution of amphibolitic lower crust with a residence time of ca. 400 Myrs. Although these adakitic rocks have depleted, mantle-like Hf isotope compositions, they are part of an existing, older crustal foundation and thus represent a process of crustal reworking, instead of crustal growth. Therefore, proportions of mantle-derived materials that contributed to the Phanerozoic crustal growth based on Hf-in-zircon signatures alone may be overestimated. Combined mineral and whole-rock radiogenic and stable isotopes are required to address this issue in the future.
AB - Rocks with “adakitic” affinities are widespread in continental collision zones, recording important information about crustal thickening. Residual garnet during partial melting of the lower crust is traditionally considered to be responsible for heavy rare earth element and Y depletion, which is a key characteristic feature of these rocks. However, magmatic garnet is rarely observed in such adakitic igneous rocks and associated protolithic eclogite is also rarely reported. Instead, amphibole is a proposed abundant phase in lower arc crust. Whilst both garnet- and amphibole-rich assemblages can act as potential sources for intracrustal intermediate-silicic melts, their distinctive fractionation of fSm/Nd and fLu/Hf should, with time, lead to different Nd-Hf isotopic compositions in high Sr/Y rocks. Here we report whole-rock radiogenic Sr-Nd-Hf and stable Fe isotope compositions of well-characterized Oligocene to Miocene (33-14 Ma), post-collisional high-K adakitic intrusions from the Gangdese belt, South Tibet. The plutons have a low alumina saturation index and depleted Dy/Yb, pointing to negligible amounts of sediment contribution. They are characterized by variable initial 87Sr/86Sr ratios (0.7048 to 0.7078), mostly negative εNd (−4.9 to +0.3), positive εHf values (+1.5 to +8.3), and variable but heavy δ57Fe values ranging from +0.04 ± 0.08‰ to +0.35 ± 0.08‰. Modeling of Fe isotopes and fractionation factors of fSm/Nd and fLu/Hf indicate an amphibolite source. A Nd-Hf isotope correlation that is quasi-parallel to the mantle array can be best explained by the evolution of amphibolitic lower crust with a residence time of ca. 400 Myrs. Although these adakitic rocks have depleted, mantle-like Hf isotope compositions, they are part of an existing, older crustal foundation and thus represent a process of crustal reworking, instead of crustal growth. Therefore, proportions of mantle-derived materials that contributed to the Phanerozoic crustal growth based on Hf-in-zircon signatures alone may be overestimated. Combined mineral and whole-rock radiogenic and stable isotopes are required to address this issue in the future.
KW - amphibole effect
KW - crustal reworking
KW - Gangdese adakitic intrusion
KW - Nd-Hf decoupling
KW - Sr-Nd-Hf-Fe isotopes
UR - http://www.scopus.com/inward/record.url?scp=85167626004&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2023.118324
DO - 10.1016/j.epsl.2023.118324
M3 - Article
AN - SCOPUS:85167626004
SN - 0012-821X
VL - 619
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
M1 - 118324
ER -