USING MAGMATIC STRUCTURES TO UNRAVEL THE GROWTH HISTORY OF MAGMA PLUMBING SYSTEMS

There is a rich history of studies examining the incremental growth of magma chambers and plutons dating back to at least the 1950's (e.g., Hamilton, 1956; Buddington, 1959). With the increased awareness of the potential complexities in magma chamber growth it is important to evaluate and use a number of tools for determining growth scenarios.

We find the following magmatic structures of particular use in this regard. (1) To determine the relative timing of events it is important to use standard structural approaches in examining crosscutting (intrusive) relationships, structures along internal contacts (e.g., those formed by stoping, mingling, mixing), and overprinting relationships including between one or more mineral fabrics. (2) To determine if frozen chambers, pulses, layers, and internal contacts formed in their present orientation(s) “paleovertical” indicators are needed with the most robust being schlieren-bounded magma tubes and pipes or plumes. Surprisingly, sinking stoped blocks and small-scale diapirs do not consistently record paleovertical. (3) To determine local growth directions, the most useful “younging indicators” are intrusive relationships, schlieren-bounded trough cut-offs, and mineral grading in layers, although it should be recognized that each of these records younging at, and formed by, a different scale/process.

The following younging and paleovertical determinations, from structures formed during mafic-felsic magma interactions, should all be used with great caution since boundary instabilities and rheologic/thermal/flow gradients can form similar structures regardless of mafic sheet/intrusion geometry and orientation: (1) magmatic “load casts” and “flame structures”; (2) compaction and filter-pressing structures; and (3) asymmetries in the development of chilled margins, mafic enclave concentrations, and of hybrid and/or mingled magma.