Description
In the Lusardi Creek area and vicinity, there is a sequence of andesitic and latitic flow and pyroclastic rock interbedded with a greater volume of massive-bedded volcanically derived fragmental rock. A small exposure of marine sedimentary rock is also present near the western end of Lusardi Creek. The fragmental units range from fine-grained sandstones to coarse-grained breccias and typically contain a wide variety of angular clasts in a tuffaceous matrix exceeding 15% by volume. Most of the fragmental units appear to have been deposited as a result of highly energetic but short-lived events followed by a quick burial which prevented reworking. The flow units occur both as layered bodies concordant with adjacent units and as discordant intrusive bodies. The rocks of the study area are interpreted to have been deposited along the flanks of a rapidly rising volcanic arc where andesite and latite flows accumulated amidst masses of sloughed volcanic debris. The close proximity of marine slates and argillites suggests that the volcanic rock may be partly marine. Petrographically, the volcanic rock is characterized by porphyritic textures where plagioclase, augite, rare hornblende, and rare orthoclase phenocrysts lie in a fine-grained groundmass containing abundant feldspar microlites. Low-grade regional metamorphism has obscured original textures and mineralogies and has created a diverse group of secondary minerals including: chlorite, epidote, serpentine, calcite, albite, quartz, pyrite, clinozoisite, and hematite. Penetration of Al-rich fluids along northwest-striking zones of weakness has produced phyrophyllite mineralization. The general structure of the volcanic and sedimentary rock is homoclinal--striking northwest and dipping northeast. In contrast, several nearly horizontal contacts have been mapped. These contacts in close proximity to steeply-dipping sedimentary beds constitute a structural discordance which has previously been interpreted to be a thrust fault. In this study, it is considered that folding is an equally plausible explanation for the discordance. Two major types of systematic folding have been inferred or observed. The first is small-scale drag folding, the axis of which strikes and dips northeast, and the second is large-scale isoclinal folding, the axial plane of which is oriented roughly N4l°W/49°N. A pronounced but discontinuous cleavage nearly parallel to bedding is interpreted to be axial-plane cleavage.
