Early Paleozoic collision-related magmatism in the eastern North Qilian orogen, northern Tibet: A linkage between accretionary and collisional orogenesis
Dong Fu1, Timothy Kusky1,?, Simon A. Wilde2, Ali Polat3,1, Bo Huang1, and Zhipeng Zhou1
1Center for Global Tectonics, School of Earth Sciences, State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
2Department of Applied Geology, Curtin University, Perth, Western Australia, Australia
3Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada
ABSTRACT
Collision-related magmatism in
accretionary-to-collisional orogens records a tectonic transition from early
subduction accretionary processes to collisional orogenesis, and also plays a
signifcant role in continental growth. Here, we
present an integrated study of feld observations, geochemistry, whole-rock
Rb-Sr and Sm-Nd isotopes, and zircon U-Pb ages and Lu-Hf
isotopes for the Laohushan mafc to felsic
magmatic rocks related to initial
collision between the Alxa terrane and the Central
Qilian block along the North Qilian orogenic
belt, northeastern Tibet. The Laohushan magmatic rocks
are dominated by quartz diorites (ca. 426 Ma), with minor
tonalites enclosing dioritic enclaves (ca. 430 Ma) and hornblendite xenoliths
(ca. 448 Ma), and some coeval dolerite dikes (ca. 427
Ma) intruded into the accretionary complex.
The quartz diorites are characterized by
light rare earth element (LREE)- and large ion
lithophile element (LILE)-enrichment but have high feld strength
element (HFSE)-depleted trace element patterns and negative initial εNd (–1.6 to
–2.9) and positive zircon initial εHf (+3.0 to +6.2) values. The dioritic
enclaves are also characterized by
LREE-enriched and HFSE-depleted patterns and have
mostly negative initial εNd (–9.2 to +0.03) but positive zircon
initial εHf (+3.0 to +5.9) values. These geochemical and isotopic features,
together with isotopic mixing
calculations, suggest that the quartz diorites were
likely derived from partial melting of the lower
crust domi nated by accreted mafc oceanic
rocks with minor sediments, whereas the
dioritic enclaves originated from underplated mantlederived magmas mixed with
crust-derived melts. The hornblendite xenoliths
have high MgO, Cr, and Ni contents,
positive Th, U, and Pb anomalies, and negative
Nb, Ta, and Ti anomalies. They have negative
initial εNd (–2.8), near chondritic zircon
initial εHf (–0.4 to +1.4) values and an Archean Nd
model age (TDM = 2.74 Ga), suggesting that the hornblendites
were likely produced by partial melting of subcontinental lithospheric mantle
peridotite that was metasomatized by subduction-related melts
beneath the Archean–Proterozoic Alxa terrane.
We propose that partial melting of the lower crust of the
early Paleozoic North Qilian orogenic belt was in response to slab breakoff
and asthenospheric upwelling during the initial stage of collisional
orogenesis. This study demonstrates that heterogeneous magma sources, involving
accretionary materials (i.e., accreted oceanic crust and sediments) and various
mantle-derived components, were mixed to form the collision-related
magmatic rocks. It also highlights the
signifcance of collisionrelated magmatism in continental growth and stabilization
of newly-assembled crust in accretionary-to-collisional
orogens.