| Summary: | 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 Ar-40-Ar-39 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 Ar-40-Ar-39 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 Ar-40-Ar-39 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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