Goldschmidt 2012 Conference Abstracts
Petrological and Sr-Nd-Os isotopic
constraints on the origin of the
Fanshan ultrapotassic complex from
the North China Craton
B. CHEN1,2*, X.L. NIU2, A.K. LIU2, AND K. SUZUKI3
1Institute
of Geology and Exploration Engineering, Xinjiang
University, Urumqi, China,
binchen@pku.edu.cn (* presenting author)
2Key Laboratory of Orogenic Belts and Crustal Evolution, Peking
University, Beijing, China
3Institute for Research on Earth Evolution, Japan Agency for MarineEarth Science and Technology, Yokosuka, Japan
The Fanshan ultramafic-syenitic complex is located on the northern
margin of North China Craton (NCC), with zircon U-Pb ages of 220 Ma. It
is a concentrically zoned complex, with syenite in the core, surrounded by
ultramafic rocks (e.g., clinopyroxenite, biotitite) and garnet-clinopyroxene
syenite, respectively, towards the rim. Apatite-magnetite ore deposits are
hosted in the ultramafic rock unit. Diopside, biotite, orthoclase, melanite,
magnetite and apatite are the major minerals, with subordinate sphene and
calcite. Petrological (e.g., cumulate textures) and geochemical data point to
the formation of the complex through fractional crystallization and
accumulation of ferromagnesian phases from a SiO2-undersaturated
ultrapotassic alkaline-peralkaline parent magma. The abundance of CaO-rich
diopside, biotite, melanite, orthoclase and magnetite of the complex suggest
high CaO, Fe2O3, and K2O contents for the parent magma. The presence of
interstitial primary calcite between the cumulus minerals in the ultramafic
rocks and the occurrence of carbonatite veins indicate a CO2-rich primary
melt. Fluorapatite is a ubiquitous euhedral phase in different rocks and
accumulates as ore-bodies, which indicates that the parent magma is
enriched in CaO, P2O5 and F. In addition, the high TiO2 contents of the
clinopyroxenes, the occurrence of melanites and the pure orthoclase
compositions of feldspars argue for the high temperature features of the
magma (probably > 1000 ℃). Rocks from the complex are highly enriched in
light rare earth elements (LREE) and large ion lithophile elements (e.g., Sr
up to 5000 ppm), depleted in high field strength elements (HFSE; e.g., Nb,
Ta, Ti), and show relatively homogeneous Sr-Nd isotopic compositions with
initial 87Sr/86Sr = 0.7051 – 0.7059 and εNd(t) = –5.8 to –5.3. The
geochemical data suggest that the Fanshan complex originated from a
phlogopite-clinopyroxenite-rich lithosphere mantle source that had
previously been metasomatized by melts from carbonated oceanic crust
above a subduction zone. Highly radiogenic Os isotope compositions of the
complex (with initial 187Os/188Os = 0.3 – 2.45) suggest that the parent
magma has been contaminated by Precambrian mafic crustal rocks during
magma emplacement in crustal levels.
The occurrence of the Fanshan complex, together with many other
ultramafic/alkaline plutons of similar ages, on the northern margin of NCC
suggests that the northern NCC entered into a large-scale extensional regime
in late Triassic after the final amalgamation between Mongolian
microcontinent and the NCC, which may signify the initiation of the
Mesozoic lithospheric thinning in the NCC.
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