VARIATION IN MINERAL NUCLEATION AND GROWTH PATTERNS NEAR TABULAR PLUTONS

Textures in rocks near a tabular pluton in the middle crust were investigated using thermal models to simulate the complex spatial/temporal variations in temperature around the pluton coupled with kinetic models of metamorphic reactions to simulate mineral nucleation and growth. Country rock permeability and proximity to the pluton determines which of three basic types of thermal regimes will affect the rocks around the pluton: 1) conductive, 2) advective and 3) a regime dominated by conductive heating shortly after intrusion, followed by advective heating.

Conductive regimes are characterized by rapid heating near the pluton followed by a longer period of cooling. The rapid heating causes substantial overstepping of reactions before many nuclei form. Once a crystal has nucleated and begun to grow, the domains of local equilibrium are small due to limited diffusive transport along grain boundaries. This favors small amounts of growth of each nucleated crystal via local metastable reactions that are isolated from each other at peak T. In a conductive thermal regime, variations in the T-t history cause porphyroblasts in the roof rocks near the center of the pluton and in the wall rocks on the end of the pluton to be larger and more abundant than in roof rocks above the end of the pluton.

Advective regimes are characterized by slow heating in rocks around the ends of the pluton followed by slow cooling. Rocks immediately adjacent to the pluton in systems with advection have two thermal peaks; an early one dominated by conduction followed by a second advective event that is much longer lived than the first peak. In models with a steep geotherm, the textures of the advective regime have more abundant and larger porphyroblasts that persist much farther from the pluton than in the conductive case. Rocks close to the pluton that experience rapid conductive heating followed by advective heating have slightly more abundant and larger porphyroblasts than those in rocks farther way that heated more slowly. For a model pluton 2.5 km thick x 15 km wide with a roof at 12km depth, rocks more than 750 m from the end of the pluton were dominated by advective heating, those closer to the end than 750 m experienced conductive followed by advective heating, and those in the roof over the center experienced conductive heating with subordinate advective effects.