40Ar/39Ar Geochronology and stable isotope geochemistry of Late-Hercynian intrusions from north-eastern Iberia with implications for argon loss in K-feldspar

The cooling history of Hercynian calc-alkalic, post-kinematic plutonic intrusions of the Montnegre massif (NE Spain) has been determined by ⁴⁰Ar/³⁹Ar analysis of two hornblendes, four biotites and eight K-feldspars (Kfs). The hornblendes have ⁴⁰Ar/³⁹Ar total fusion ages of 291Dž Ma and define the magmatic cooling of the basic and oldest structural intrusions. The biotites from the acid and intermediate rocks have ⁴⁰Ar/³⁹Ar plateau ages of 285Dž Ma, which date cooling of the intrusions through argon closure in biotite. The ⁴⁰Ar/³⁹Ar ages of the K-feldspars vary widely, ranging from 276-191 Ma. A correlation between K-feldspar ⁴⁰Ar/³⁹Ar total fusion age and several other features such as structural state, microstructures indicated by obliquity and, to some degree, optically visible perthites, is consistent with post-crystallisation partial argon loss in the K-feldspars. The 'D values of the biotites also correlate with age and chlorite contents, but this is not so for the '¹⁸O values of either feldspar or quartz. We infer that most microtextural changes occurred during cooling of the batholith, but a possibly Mesozoic, late disturbing hydrothermal event of weak intensity and with only minor fluid circulation must have occurred. This event provoked significant argon loss in the most structurally complex K-feldspars and is recorded in the hydrogen, but not oxygen, isotope data.