Geology Reference
In-Depth Information
Neogene volcanic rocks are also present in the southern part of Tibetan Plateau
(Coulon et al. 1986). The central and southern portion of Tibet is divided into three
blocks:
(1) The Lhasa block,
(2) The Quantang block and
(3) The Kunlun block.
The Yarlung-Tsangpo ophiolite suture separates the Lhasa block from the Indian
Plate (Fig.
4.2
). The Lhasa block is separated from the Quantang block by the
Bangong Nujian suture zone, which is tectonically dismembered. The Kunlun block
is separated in the south from the Quantang block, and in the north from the
Eurasian plate by two suture zones. The northern part of the Lhasa block is con-
stituted of the Precambrian gneissic basement rocks overlain by Jurassic sedi-
mentary rocks, which in turn are overlain by rocks of lower Cretaceous series.
Mesozoic volcanism took place at Gyanco, Barda, Nagqu and Anduo during
Cretaceous. Large volumes of basalt, andesite and dacites are observed in these
places, whereas ignimbrites are rare. East of Lhasa, volcanic rocks of supposedly
Triassic age constitute a thick (1,400 m) calc-alkaline suite, composed of ignimb-
rites, andesite and porphyritic basalts. Towards south, there is a folded Cretaceous
sedimentary formation, known as Takena formation, which is uncomfortably
overlain by a volcanic sequence, known as the Lingzizong formation, which is
made up of andesite and abundant rhyolitic ignimbrites.
The Lingzizong volcanics are affected by a Pleistocene rift structure towards
northeast of Yangbajin. Along the eastern border of this rift the andesite and rhy-
olite are associated with phonolitic tephrites and trachytes. The plutonic block of
Ganjdese lies southeast of the Lhasa block. Biotite and sanidine extracted from a
trachyte gave ages of 50
1.1 Ma, respectively. The phonolitic
tephrite and alkali trachyte are characterized by high alkali contents
[(Na
2
O+K
2
O) > 1, K
2
O/Na
2
O > 1)]. Compared to the calc-alkalic rocks, they are
richer in Rb, Y, Zr and Nb. Tephritic and trachytic lavas have also been obtained in
the northern border of the Gangdese volcanic chain indicating a northward K
2
O
enrichment. They are similar to rocks of Aeolian Island Arc and Azerbaijan potassic
complex of Iran.
The tephrites are characterized by phenocrysts of analcite (about a centimetre
across) and pseudomorphs after leucite phenocrysts, plagioclase (An
56
-
53
), titeno-
magnetite and altered olivine, set in a groundmass of augite (Ca
47
Fe
17
Mg
36
), Fe
±
1 Ma and 81.1
±
Ti
oxide and abundant sanidine (Or
60
). The trachytes are characterized by the presence
of sanidine, biotite, Fe-Ti
-
Ti oxide, scarce olivine (oxidized), apatite with or without
plagioclase (An
60
-
40
), augite (Ca
40
Fe
22
Mg
38
) and sanidine (Or
65
). Biotite may
occur in the groundmass as a late stage product with edenitic amphibole in the
trachytes and tephrites. Analyses of Yangbajin K-rich rocks are summarized in
Table
4.1
.
-
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