Geological events recorded by automorphic feldspars and associated minerals in Cretaceous Ashua limestones and the metamorphic zone of the Chejol dacite intrusion, Peru

Abstract Geological field observations, coupled with laboratory methods including X-ray diffraction, scanning electron microscopy with energy-dispersive spectrometry, electron microprobe, Raman spectroscopy, and cathodoluminescence, were used to solve the complex problem of the origin of automorphic feldspars and coexisting silicates and aluminosilicates dispersed in the Late Cretaceous Ashua limestones and their metamorphosed crystalline varieties in southern Peru. Petrographic and geochemical data reveal at least three distinct generations of feldspar. The earliest generations of consistent crystallographic orientation (albite A cores and B rims), commonly associated with sericite and adularia, are interpreted as magmatic to postmagmatic in origin. They derived from pyroclastic material of the Early Cretaceous Matalaque Formation within the West Peruvian Trough. Albite grains, introduced as volcaniclastic detritus, are found both in the siliciclastic deposits of the Ashua and/or Chilcane formations and sporadically in restricted, evaporitic marine carbonate environments where celestite, barite, and opal A also precipitated. The outermost regeneration rim (albite-oligoclase composition; generation C), exhibiting distinct crystallographic orientations, conferred the automorphic habit to the feldspars within the contact aureole of the Paleogene Chejol hypabyssal dacite intrusion. It crystallized within metasomatic assemblages representative of skarns, alkaliamphibole, and aceite-type metasomatites, as well as propylitic facies, which formed under conditions ranging from ~500 °C to 150 °C at near-neutral pH (~8). As the main cations involved in the threephased processes are Na, K, Mg, and Ca, this feldspar-C generation was associated with cogenetic minerals in crystalline limestone hosts. Al-rich titanite, Mg-chlorite, talc, and authigenic quartz characterize the earliest hydrothermal pulse. The subsequent hydrothermal episode led to the zeolitization and albitization, followed by the formation of quartz pseudomorphs after stilbite and albite.