Geochemical Constraints on the Petrogenesis of Cretaceous to Paleocene Granitoids and Volcanic Rocks from SE Tibet
Date Issued
2007
Date
2007
Author(s)
Lin, I-Jhen
DOI
zh-TW
Abstract
Northward subduction of the Neo-Tethyan oceanic lithosphere (新特提斯洋板塊) beneath South Asia gave rise to an Andean-type convergent margin with arc magmatism starting since at least the early Jurassic (>190 Ma) and lasting until the Eocene (~45 Ma). Compared with the Gangdese (岡底斯) Batholith, i.e., the largest intrusive exposure thus formed in the southern Lhasa (拉薩) terrane, central part of southern Tibet, the temporal and spatial distribution of the arc magmatic product in its eastern/southeastern extension remains poorly understood. This thesis reports the first systematic investigation of granitoids from the Bomi (波密) to Chayu (察隅) area, between ~95.5°E and 97.5°E, and ~30°N and 28.5°N, the southeastern Tibetan plateau. From NW to SE, four batholiths, namely, Achakung (阿扎貢), Temula (德母拉), Chayu (察隅) and Shama (沙馬), were investigated. Moreover, a comparative study of early Cretaceous volcanic rocks from Ranwu (然烏) in the same region was also performed.
Zircon U-Pb age data show that these granitoids were emplaced in the Cretaceous (~133-110) and Paleocene (~60Ma), respectively. In addition to the widespread calc-alkaline granitoids, A-type granites and highly fractionated S-type granites are observed in the Temula and Chayu batholiths, respectively. All the granitoids are characterized with depletions in the high field strength elements (HFSE), enrichment in the large ion lithophile elements (LILE) and light rare earth elements (LREE), and show spidergrams similar to those of typical arc magmas from subduction zones or active continental margins. The A-type granites, a rock type rarely reported in the convergent margin settings, are characterized by high alkalis (Na2O+K2O= 7.1-8.3 wt.%; K2O= 4.5-5.5 wt.%; Rb= 491-713 ppm), high Ga/Al, high Fe2O3 T/MgO, and high HREE [La= 20.1-54.5 ppm; (La/Yb)n= 1.5-4.9], coupled with large negative Eu anomalies [(Eu/Eu*)= 0.02-0.04], and low Ba (10-46 ppm) and Sr (6.5-11.5 ppm) contents. The granitoids from the Bomi to Chayu area, as a whole show heterogeneous Sr and Nd isotope compositions, with εNd(T) values varying from -1.0 to -13 and ISr ratios from 0.705 to 0.746, which correspond in general to those of S-type granitoids from the northern plutonic belt of the Lhasa terrane [εNd(T)= -4 to -13, aged ~150-85 Ma; Wen, 2007] and differ markedly from those of the I-type Gangdese Batholith [εNd(T)= +5.3 to +2, aged ~103-80 Ma; εNd(T)= +5.3 to -0.5, aged ~65-45 Ma; Wen, 2007].
The Ranwu volcanic rocks, have a wide range of compositions from basaltic to dacitic (SiO2= 46-69 wt. %), are calc-alkaline in nature. Relative to the neighboring granitoids, the volcanic rocks have higher εNd(T) (+3 to -6) and lower ISr ratios (0.705 to 0.707) that suggest involvement of the lower continental crust to have played an important role together with fractional crystallization in the petrogenesis. The overall Sr-Nd isotope compositions of the granitoids, furthermore, suggest three source components in the magma generation, namely, the mantle wedge, the lower continental crust and the upper continental crust. Consequently, we propose a two-stage magma differentiation model involving (1) deep differentiation and crustal contamination of mantle-derived mafic liquids that intrude/underplate in the lower part of the continental crust around the Moho and (2) additional differentiation and upper crustal contamination of such differentiated liquids as they rise and install in shallow-level magma chambers. While the former scenario explains how the Ranwu volcanic rocks were generated, the latter accounts for the emplacement and petrochemical characteristics of the Bomi-Chayu batholiths. The shallow intrusions, moreover, may have triggered low-pressure (P≦4 kbar; Patino-Douce, 1997)
partial melting of calc-alkaline basement rocks, e.g., those atop the magma chambers, and hence produced the A-type granites in this part of the Neo-Tethyan subduction zone.Based on the age, compositional and isotopic constraints, it is concluded that the Bomi-Chayu batholiths probably correlate with the northern plutonic belt of the Lhasa terrane. In other words, the eastern/southeastern extension of the Gangdese Batholith, if existed, should be found further south within the Indian national territory that, however, still awaits future explorations.
Subjects
藏東南
拉薩陸塊
白堊紀
古新世
新特提斯洋板塊
北部岩漿岩帶
岡底斯岩體
A型花崗岩
然烏火山岩
SE Tibet
Lhasa terrane
Cretaceous
Paleocene
Neo-Tethyan oceanic lithosphere
Northern Plutonic Belt
Gangdese Batholith
A-type granites
highly fractionated S-type granites
Ranwu volcanic rocks
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-96-R94224201-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):eda663e080a52bbbc839b39050cf0a2d