A COMPREHENSIVE U-PB, SM-ND, RB-SR AND 40AR-39AR GEOCHRONOLOGICAL STUDY ON GUIDONG GRANODIORITE, SOUTHEAST CHINA - RECORDS OF MULTIPLE TECTONOTHERMAL EVENTS IN A SINGLE PLUTON

被引:49
作者
LI, XH
机构
[1] Guangzhou Branch, Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou
关键词
D O I
10.1016/0009-2541(94)90192-9
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
A complicated tectonothermal history of the Guidong Granodiorite from the Huanan Fold Belt has been revealed by using combined dating techniques. U-Pb single zircon and whole-rock Nd model ages suggest that the granodiorite was derived from an ancient crustal source which was probably composed of a mixture of 1.2-2.4-Ga crustal materials with a Nd model age of 1.6 Ga. The granodiorite emplacement age of 42 7 +/- 3 Ma is defined by concordant U-Pb single-zircon analysis. Plagioclase, biotite, hornblende, apatite, sphene and whole rock yield an internal Sm-Nd isochron age of 428 +/- 12 Ma, consistent within the analytical errors with the U-Pb zircon age. Due to the presence of inherited zircon, however, the zircon data point deviates from the Sm-Nd isochron. Four whole rocks and two mineral separates (plagioclase and hornblende) yield a Rb-Sr isochron age of 422 +/- 14 Ma, which is coincident with zircon U-Pb and Sm-Nd isochron ages. Hornblende 40Ar-39Ar step-heating analysis gives a main plateau age of 371.4 +/- 1.3 Ma, which is considerably younger than the crystallization age. This age is interpreted as recording a post-crystallization folding event which produced the prominent unconformity between pre-Devonian and Middle Devonian strata, and resulted in the deformation of all major minerals of the granodiorite. Since most deformed minerals have remained closed to Sm-Nd and Rb-Sr isotopic migration since emplacement of the granodiorite, it can be suggested that this folding event took place without significant fluid circulation and contemporaneous metamorphism. A low-temperature heating step of hornblende yields an apparent 40Ar-39Ar age of 208.5 +/- 2.0 Ma which may or may not have a connection to the minimum age of the Huanan Indosinian Orogeny. A biotite 40Ar-39Ar age spectrum yields much younger apparent ages of 145 and 92 Ma which are coincident with two important Yanshanian magmatic episodes. A complicated tectonothermal history of the Guidong Granodiorite from the Huanan Fold Belt has been revealed by using combined dating techniques. U-Pb single zircon and whole-rock Nd model ages suggest that the granodiorite was derived from an ancient crustal source which was probably composed of a mixture of 1.2-2.4-Ga crustal materials with a Nd model age of 1.6 Ga. The granodiorite emplacement age of 42 7 +/- 3 Ma is defined by concordant U-Pb single-zircon analysis. Plagioclase, biotite, hornblende, apatite, sphene and whole rock yield an internal Sm-Nd isochron age of 428 +/- 12 Ma, consistent within the analytical errors with the U-Pb zircon age. Due to the presence of inherited zircon, however, the zircon data point deviates from the Sm-Nd isochron. Four whole rocks and two mineral separates (plagioclase and hornblende) yield a Rb-Sr isochron age of 422 +/- 14 Ma, which is coincident with zircon U-Pb and Sm-Nd isochron ages. Hornblende 40Ar-39Ar step-heating analysis gives a main plateau age of 371.4 +/- 1.3 Ma, which is considerably younger than the crystallization age. This age is interpreted as recording a post-crystallization folding event which produced the prominent unconformity between pre-Devonian and Middle Devonian strata, and resulted in the deformation of all major minerals of the granodiorite. Since most deformed minerals have remained closed to Sm-Nd and Rb-Sr isotopic migration since emplacement of the granodiorite, it can be suggested that this folding event took place without significant fluid circulation and contemporaneous metamorphism. A low-temperature heating step of hornblende yields an apparent 40Ar-39Ar age of 208.5 +/- 2.0 Ma which may or may not have a connection to the minimum age of the Huanan Indosinian Orogeny. A biotite 40Ar-39Ar age spectrum yields much younger apparent ages of 145 and 92 Ma which are coincident with two important Yanshanian magmatic episodes.
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页码:283 / 295
页数:13
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