Temporal variation of titanite morphology and chemistry in a long-lived shear zone: The Clarke Head syenite in the Minas Fault Zone, Nova Scotia

Abstract

The mineral chemistry of titanite has been studied in hydrothermally altered syenite preserved as blocks in a salt wall within a regional intracontinental shear zone. The study focusses on the controls on temporal variation in REEs, Zr, Nb, U and Th. Several generations of titanite are recognised on the basis of morphology, chemistry, and relationship to other secondary minerals. Magmatic euhedral to subhedral titanite has Ti and Zr contents characteristic of magmatic titanite in other studies, but does not have a magmatic abundance of REE. Rather, it is 102–104 times depleted in LREE and only slightly depleted in HREE. The leaching of LREE began during a regionally recognised phase of potassic alteration, as amphibole and biotite in gabbro and diorite recrystallized along shear zones and released K, Ca, and F. Later, some 15 Ma after emplacement, the alkali feldspar in the syenite was largely altered to scapolite and analcime by interaction with adjacent halite and gypsum deposits. Titanite formed at that time, from dissolution-reprecipitation reactions in large rutile crystals, is less depleted in LREE, as a result of the dominance of Cl− in hydrothermal waters. Following that event, F−-dominated hydrothermal waters prevailed again, resulting in more LREE depletion in previously formed titanite and in neoformed hydrothermal titanite. At that time, there was also depletion in Zr and U. The trace elements in hydrothermal titanite are thus very informative on the hydrothermal history of the rocks. A combination of morphology and trace-element signature allows different hydrothermal stages to be distinguished.