Evolution of phyllosilicates through diagenesis and low-grade metamorphism in a prograde sequence of pelitic rocks from southern New Zealand.

Abstract

Phyllosilicates in a prograde sequence of pelitic rocks (zeolite facies to biotite zone of the greenschist facies) from southern New Zealand have been studied using transmission and analytical electron microscopy (TEM and AEM), scanning electron microscopy and X-ray diffraction (XRD) to determine the relations among mineral reactions during diagenesis and metamorphism. In zeolite facies rocks, large detrital phyllosilicates coexist with fine-grained matrix clay minerals, which form at different diagenetic stages and occur intimately intergrown or mixed-layered on a submicron scale. In contrast with the dominant smectite in mixed-layer illite/smectite in bentonite, authigenic illite with little smectite occurs in closely associated siltstones and analcimized tuffs at the same burial depth. Thus, the rate of the smectite-to-illite transition is much slower in bentonite, implying that reaction rates are controlled by several interacting factors rather than by temperature alone. Detrital biotite has been altered to chlorite/corrensite. Pyrite and sphalerite inclusions within chlorite/corrensite have been demonstrated to form by reaction of Fe from biotite and S, Zn from pore fluids during biotite alteration. Two new celadonite-type minerals, ferroceladonite and ferroaluminoceladonite, have been characterized from heulanditized tuffs. Complete or nearly complete solid solution occurs among the four end-members of the celadonite family. At higher grades, detritus undergo dissolution while metamorphic phyllosilicates crystallize or recrystallize. Defect-rich 1$M\sb{d}$ illite changes to defect-free 2$M\sb{l}$ phengite, and their average crystal thicknesses increase with increasing metamorphic grade, as demonstrated by similar trends from direct TEM measurements and XRD profile analyses. Correlation of TEM-measured crystallite sizes with illite crystallinity (IC) data indicates that IC may serve as a general parameter for estimating the degree of diagenesis/metamorphism only on a relative basis, and IC is not sensitive to metamorphic grade under greenschist facies conditions. Chemical and textural relationships suggest that stilpnomelane forms in pumpellyite-actinolite facies rocks by reaction of chlorite, phengite, and hematite, and that biotite forms in biotite zone rocks by consumption of stilpnomelane. Evolution of phyllosilicates is related to decrease of surface and strain energy and reduction of chemical free energy, following the Ostwald step rule to approach stable equilibrium during metamorphism.