ABSTRACT
Cretaceous and Lower Tertiary sedimentary rocks are well exposed
in the Tingri-Gyangtse area, tectonically belonging to the central
Tibetan Himalayas, to the south of the Indus-Yarlung-Zangbo
suture. The E-W trending Gyirong-Kangmar thrust divides them into
two subzones (north zone and south zone) of different lithological
compositions.
The Cretaceous Tianba flysch in the Tianba-Jiabula region is
correlative with the Giumal Group sandstone in Zanskar,
northwestern Himalayas. There are significant amounts of
chrome-rich spinels in turbiditic sandstones from the upper part
of Tianba Flysch, which might suggest ophiolite derivation and a
Cretaceous ophiolite obduction event on the northern Indian
continental margin in southern Tibet. However the compositional
range of these detrital spinels closely matches that of spinels
from intra-plate basalts. About 5% of the spinels contain melt
inclusions. The compositions of melt inclusions correlate well
with those of host spinels. Melt inclusion geochemistry also
suggests a source of hotspot basalts. It is concluded that the
Rajmahal volcanics were the source for these Cr-rich spinels.
The continuous Cretaceous to Lower Eocene marine sedimentary
series in the Gamba and Tingri areas suggest that the Indian-Asian
collision must have started after the deposition of the youngest
marine shelf sediments. Petrographical analysis of sandstones
reveals that the monocrystalline quartz grains of cratonic origin
are dominant in the Paleocene Jidula Formation; in contrast there
are significant amounts of immature framework grains with a
distinct ophiolitic and volcanic arc influence present in the
Eocene Youxia Formation and the younger Shenkeza Formation.
Geochemistry in both sandstones and shales complement the
petrographic data indicating that the source of the Jidula
Formation primarily consisted of quartzose basement rocks, while
the Youxia and Shenkeza Formations are mainly derived from the
uplifted Gangdese arc-trench system. The compositions of Cr-rich
spinels in the Youxia and Shenkeza sandstones are similar to those
from fore-arc peridotites, most likely from the arc and ophiolite
rocks along the Yarlung-Zangbo suture to the north. No spinels
have been observed in the Jidula sandstones. Therefore the early
Tertiary detrital sediments in Tingri record a marked change in
provenance in the early Tertiary, which indicates that the onset
of India-Asia collision was at ~47 Ma in southern Tibet.
Zhu, B., 2003. Sedimentology, petrography, and tectonic
significance of Cretaceous to lower Tertiary deposits in the
Tingri-Gyangtse area, southern Tibet. Unpublished PhD
dissertation, State University of New York at Albany. 213 pp.,
+xi;
University at Albany Science Library call number: SCIENCE
MIC Film QE 40 Z899 2003 Z58
Copies of this PhD dissertation can be ordered
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