Northern Tibet volcanic field is a volcanic field[lower-alpha 1] in China.
The volcanic field lies in the Quiangtang plateau, in northern Tibet and the Kunlun mountains. The high altitude, bad terrain and weather make geologic investigations in the region difficult.[2] The volcanic field partly covers the Kangtuo formation.[3]
The Bamaoqiongzong (34°00′N 88°45′E[lower-alpha 2]), Yongbohu and Qiangbaqian (35°30′N 88°15′E) fields featured Hawaiian eruptions that have formed lava flows. Later erosion has reduced these to flat topped conical hills and table mountains.[4] Other volcanic centres are the cones of Heiguotou (34°15′0″N 90°45′0″E), Kushuihuan-Beishan (34°15′0″N 90°0′0″E) and Yuyiehu (33°15′0″N 90°0′0″E). In addition, the Kekexili (35°51′0″N 91°42′0″E) caldera is considered part of the field.[1] This caldera has a diameter of 4 kilometres (2.5 mi). A volcano named "Debussey" is supposedly also located there.[5]
Bamaoqiongzong covers a surface area of 300 square kilometres (120 sq mi).[6] It forms a 5,400-metre-high (17,700 ft) peak with column-jointed rocks at its top.[6] Two craters lie on this peak, one 230 metres (750 ft) wide and 20 metres (66 ft) deep on the northeastern side with a breach southeast and another less well preserved one on the southeastern side.[7] It is surrounded by lava domes[6] and lava sheets. Originally likely over 300 metres (980 ft) thick, now they reach only several 10 metres (33 ft).[7]
Yongbohu features five vents,[1] the principal one has a height of 150 metres (490 ft). One has the shape of a dragon and another one is conical. Quiangbaqian covers a surface of 55 by 70 kilometres (34 mi × 43 mi) and features lava tube and fissure fed lava flows as well as one 200-metre-high (660 ft) volcano.[5] These volcanics are also known as Yulinshan formation.[3] Sometimes, the chronologically, petrologically and geographically distinct Hoh Xil volcanics are grouped with these fields.[8]
Bamaoqiongzong, Yongbohu and Qiangbaqian have erupted vitrophyric rocks. Minerals encountered in the vulcanites include andesite, trachyandesite and dacite containing augite,[4] and are subalkaline.[lower-alpha 3] Bamaoqiongzong is potassic,[10] it also contains phonolite and its minerals include aegyrite, alkali feldspar, analcite,[4] foidite,[1] leucite, noselite and titanaugite. This differences may be caused by the position of the centres with respect to the north Tibet block; the last two centres are located on its margin and the first one inside the block.[4]
The Bamaoqiongzong, Yongbohu and Qiangbaqian volcanoes appear to be of Quaternary age, seeing as their deposits lie above Pliocene-Pleistocene rocks.[4] Lava flows from Bamaoqiongzong lie on Quaternary lake deposits.[1] Argon-argon dating has yielded ages of 30-26, 18 and 15-14 million years ago for the fields respectively however.[10] Potassium-argon dating of Bamaoqiongzong has yielded ages ranging from 20 to 28.6 million years ago.[2]
There is no evidence for Holocene eruptions. A satellite photo supposedly showing activity from[1] a crater (35°51′N 91°42′E) on a 1.5-kilometre-wide (0.93 mi) cone on the ring fault[5] of the Kekexili caldera in 1973 was later discarded.[1]
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