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incompatible element depleted anhydrous lherzolite a sthenospheric mantle source
and an enriched, amphibole-phologite (apatite-) bearing lherzolite continental
lithospheric mantle source. They proposed that the basanites are the products of
very low degree partial melts (<1 %) of this source under high extension strength.
4.1.2 K-Rich Volcanics from Yangbajin Rift, Tibet
The Tibetan high plateau in the Himalayan and Karakoram Ranges are product of
continuous collision between India with the southern margin of Eurasia plate during
past 15 Ma (Coulon et al. 1986). Potassic and ultrapotassic lava
ows and pyro-
clasic rocks from the volcanic
fields, south of Xungba and north of Borgba
(Fig.
4.2
). The ages of the rocks range between 17 and 25 Ma. The K
2
O content of
these rocks vary from 5.98 to 6.86 wt% and the K
2
O/Na
2
O ratios ranges between
2.38 and 6.58, average being 3.27. The ultrapotassic rocks are characterized by
phenocrysts of phlogopite, clinopyroxene and microphenocrysts of apatite and
titanomagnetite set in a pale-brown glassy matrix. In some samples sanidine,
clinopyroxene, Ti-phlogopite and olivine are present. Their
87
Sr/
86
Sr ratio varies
between 0.7172 and 0.7220 and
143
Nd/
144
Nd ratio ranges between 0.51190 and
0.51200, whereas the ranges of
207
Pb/
204
Pb and
208
Pb/
204
Pb vary respectively
between 15.68
15.72 and 39.42
39.60.
-
-
81
82
to Shiquanhe
to Gertse
Tsota (5690m)
Bangha
Xungba
32
Gongqen
32
Gar
Labru
LHASA BLOCK
Gegar
KF
Jarga
Indus
Melange with flyschoid
Upper Cretaceous matrix
Miocene high-K
calc-alkaline volcanic rocks
thrust
Ta ken a Fo r m at io n,
mostly volcanic rocks
Transhimalaya
Batholith
Miocene ultrapotassic and
potassic volcanic rocks
Linzizong Formation,
mostly volcanic rocks
Quaternary,
Neogene sediments
Fig. 4.2 The K-rich mafic rocks from Tibet (after Coulon et al. 1986)
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