Application of ICP-MS to the study of Transhimalayan petrogenesis
Date Issued
2006
Date
2006
Author(s)
Chu, Mei-Fei
DOI
en-US
Abstract
Recent progress on the development of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) makes precise in-situ measurement of trace element concentrations of accessory minerals in igneous rocks possible and thus the method a powerful tool for studying the complex processes that form and modify the host magmas. In this thesis, focused on the application of LA-ICP-MS to the study of Transhimalayan petrogenesis, in-situ trace element and isotope data of zircon and apatite separates from different types of Transhimalayan rocks in the Lhasa terrane, southern Tibet were carried out. These rocks include: (1) I-type granitoids from the Gangdese batholith cropping out in the southern part of the Lhasa terrane, (2), S-type granitoids from the Nyainqentanglha or northern magmatic belt in the Lhasa terrane, and (3) post-collisional adakites emplaced in the Gangdese belt.
This thesis, furthermore, contributes to the set up of the first LA-ICP-MS system at Department of Geosciences, NTU, composed of a New Wave LUV213 laser and an Agilent 7500s quadruple ICP-MS. The contribution includes: (1) establishment of the routine analytical procedures of rock samples (powder and glass bead) using solution method, (2) successful link the laser with the ICP-MS for in-situ analysis of trace elements in apatites, and (3) collection of knowledge that forms a basis for setting up the new LA-ICP-MS system, attached with a New Wave UP213 laser, at NTU that can perform zircon U-Pb dating as well as in-situ trace element measurements.
ZIRCONS’ Hf isotope ratios can be used in much the same way as whole-rock Nd isotopes. They, furthermore, often record “hidden” information that allows more detailed studies of the magma generation processes. Based on zircon Hf isotope data obtained in this study, together with associated U-Pb ages, the following conclusions regarding Transhimalayan petrogenesis are reached: (1) There are significant variations in Hf isotopes of magmatic zircons, up to ~15 ε-units in some samples, suggesting magma mixing to be a common process; (2) A “hidden” DM (depleted mantle) component, with εHf(T) values up to +19.8, is identified to be prevalent in the Gangdese magmatic zircons. This DM-type component has never been revealed by any whole rock isotope analysis; (3) While the “conventional” Gangdese magmatism has been known as most active in the Cretaceous and Paleogene, this study identifies a new magmatic episode within the Gangdese belt that occurred in Early Jurassic time resulting from the long-lasting Neo-Tethyan subduction; (4) The S-type granitoids of the northern magmatic belt contain abundant inherited zircons aged from ca. 188 to 210 Ma, in which a crustal component that shows εHf(T)= -3.9 to -13.7 and TDMC model ages of ca. 1.4- 2.1 Ga is identified. This implies a major stage of crustal growth in Proterozoic time and remelting of the crustal material in Early Jurassic time; (5) The Oligocene-Miocene adakites contain magmatic zircons that show similar Hf isotope compositions to the Cretaceous-Paleogene Gangdese batholiths, providing a key constraint that allows evaluation of the nature and timing of crustal thickening in southern Tibet owing to the India-Asia collision; and (6) As a whole, the Hf isotope information observed in zircons from the above Transhimalayan rocks demonstrates a temporal variation in εHf(T) values, and thus TDMC ages, that suggests multiple stages of orogenic or crustal formation events.
APATITES from different types of igneous rocks generally reveal significant variations in the abundance level of minor and trace elements. In this study, EPMA and LA-ICP-MS were used to determine the major and trace element concentrations, respectively, of apatites from Transhimalayan granitoids. The results indicate that F, Mn, Sr and REEs in apatites generally show good correlations with compositions of their host magmas and thus have high potential to be utilized as petrogenetic tracers. More specifically, F and Mn contents in apatites are covariant with the aluminosity (or ASI values) of the host rocks so that can be used as an indicator for magma differentiation. Combining with Sr and REE data, which show significant variations in apatites from different rock types, these elements may be furthermore used to construct “discrimination diagrams” for more detailed investigations of complex petrogenetic processes such as magma mixing and compositional heterogeneity.
Subjects
電漿質譜術
微區
西藏
鋯石
鉿同位素
磷灰石
拉薩地塊
ICP-MS
in situ
Tibet
zircon
Hf isotope
apatite
Lhasa terrane
Transhimalaya
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-95-F88224109-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):1d30d20c52fe03dbca76b6f935410d8e