AN EVALUATION OF YOUNGING DIRECTION AND PALEOVERTICAL INDICATORS IN MAGMA CHAMBERS

A central issue in evaluating the spatial and temporal evolution of magma chambers is the ability to determine younging directions and/or paleovertical in these chambers. Understanding the dynamics of an incrementally constructed intrusion can be difficult using only geochemistry and geochronology because other subtleties in chamber processes may mimic chamber growth. Combining these tools with careful documentation of younging and/or paleovertical can help evaluate the spatial and temporal evolution of incrementally emplaced magma chambers. Furthermore, paleovertical indicators along with thermobarometric determinations are particularly useful for paleomagnetic studies.

Younging indicators commonly noted in plutons include 1) trough cut-offs in schlieren layering, 2) grading of mineral in schlieren layering, 3) load casts and flame structures, 4) asymmetry of chilled margins, 5) asymmetry in mafic enclave concentrations, 6) asymmetry of hybrid magma and/or mingled magma. The most reliable indicator are trough cut-offs in schlieren layers, comparable to crossbedding in sedimentary layers. Some of these structures have been interpreted to reflect sequential injection of mafic magma into a felsic chamber to form gently dipping sheets and thus a pluton stratigraphy (e.g. 3-6; Wiebe and Collins, 1998). Indicators associated with mafic-felsic magma interaction should be used with caution because factors including rheologic contrasts, boundary flow and thermal gradients between mafic-felsic magma intrusions can form these structures regardless of mafic sheet/intrusion geometry and orientation.

Paleovertical indicators recognized include pipes/tube structures, diapirs and asymmetrical deflection of layering and/or mineral foliation by stoped blocks. The most reliable indicators are pipe and tube structures interpreted to result from buoyancy driven rise of magma. Other paleovertical indicators should be used with caution as their modes of formation are non-unique.

Most importantly, consistency within spatial domains in a pluton using multiple indicators should be sought and patterns of chamber growth should be based on all available data as well as from geochronologic, geochemical and thermobarometric studies.