Amphibole crystallization conditions as record of interaction between ultrapotassic enclaves and monzonitic magmas in the Glória Norte Stock, South of Borborema Province

Abstract

The Glória Norte Stock (GNS) is made up of predominantly porphyritic biotite- amphibole-bearing quartz monzonite, enclosing a large number of microgranular mafic enclaves (MME). The GNS MME are fine-grained rocks with rounded and ellipsoid shapes that show sharp and gradational contacts with the host rocks, suggesting they coexisted with the host monzonite as magmas. The studied amphibole crystals of the two rock types are calcic and correspond to pargasite, edenite, and magnesium-hornblende. Their compositions are influenced by substitutions involving Al³⁺, Na⁺, Fe²⁺, Si⁴⁺ and Mg²⁺ ions, reflecting the decrease in temperature and the increase in oxygen fugacity. They present low Ti and Al contents, and varied Si content (6.2-7.7 apuf). The mg# values range from 0.48-0.84. Pressures for the crystallization of the MME amphiboles vary between 2.6 and 7.8 kbar and the temperatures of solidus and liquidus were estimated between 600-659 and 887-908ºC, respectively. The early MME amphibole crystals with lower Si and Mg content were formed under high pressure (7.8 kbar) and temperature (908 °C). The presence of amphiboles with 862ºC and 5 kbar in MME and GNS reflects that the interaction between these magmas occurred at 18 km of depth.

KEYWORDS:
mineral chemistry; amphibole thermobarometry; Sergipano Orogenic System