中生代以來華南地區之熱歷史及大地構造演化研究(3/3)
Date Issued
2003
Date
2003
Author(s)
陳正宏
DOI
912116M002032
Abstract
The Early Yanshanian granitoids in S
China have long been suggested to represent
a subduction-related orogeny in Jurassic.
Biotite granite is the predominant lithology
particularly for large plutons, and gabbroic
and syenitic rocks are present scarcely as
small igneous bodies. 40Ar/39Ar hornblende
ages yield ~160 Ma and biotites yield
160-155 Ma for these granites. Similarities of
ages between these two minerals indicate
rapid cooled thermal histories of the plutons,
thus these ages can be used to approximate
the emplacing time. Using the same methods,
ages obtained are 135-130 Ma for the syenitic
rocks.
Geochemically, biotite granites belong to
high-K calc-alkaline affinity and syenites are
shoshonitic rocks. In terms of Sr-Nd isotopic
compositions, more depleted syenites are
similar to gabbros, suggesting that they could
originate from the same source. On the
other hand, biotite granites exhibit much
more enriched signatures with Sri values and
εNd(T). The εNd(T)–Sri isotopic arrays
constructed from these Early Yanshanian
intrusives could be derived from
two-component magma mixing of a primitive
component (depleted mantle) with an
evolved component (S China old crustal
basement). We suggest that biotite granites
are the partial melts of the lower crust that
has been modified in the Indosinian orogeny,
and mixed with the underplated basalts;
whereas syenitic rocks are basically the
2
partial melts of the preceded underplated
basalts.
With the short timespan of the Early
Yanshanian magmatism peaked at 165-155
Ma, extraordinary heat supplies other than
the subduction system are necessary to
account for the regional distribution of
granitoids in S China. Alternatively, this
may need a tectonic process that is related to
the large-scale intraplate lithospheric
extension. As for the presence of later
syenitic rocks, further local extension may be
an explanation. Therefore, the lack of
syn-orogenic characteristics on the Early
Yanshanian magmatism is hard to support
the Jurassic orogenesis in S China.
Publisher
臺北市：國立臺灣大學地質科學系暨研究所
Type
report
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