Estimating the Duration of Short Igneous Events using Diffusion Chronometry and Contact Metamorphism

Abstract:

We studied quartz phenocrysts from acidic volcanic rocks of the Jurassic El Quemado Complex (Patagonia, Argentina) formed during the Gondwana break-up. Cathodoluminesence (CL) images display simple oscillatory zoning of quartz from rhyolite flows, while those from pyroclastic rocks show oscillatory zoning and resorption/re-growth phenomena. Hence effusive magma likely rose directly from the source area to the surface, while pyroclastic rocks experienced storage in a mush, with heating or pressure cycles before eruption. Quantitative Ti analysis with a IMS 1280 SIMS document only very slight variations between zones. NanoSIMS analyses allow determining Ti profiles with high spatial resolution. CL and Ti profiles only partially match, indicating additional factors determine the CL signature. Diffusion modeling of Ti-profile in quartz yields residence times of 2.4-3.4 years for the flows, and 0.4-20 years for the pyroclastics.

The lifetime of a magma ascent zone – a feeder zone – can be estimated by thermal modeling of the contact aureole. The Western Adamello Tonalite (Italy) provides an excellent example. Carefully mapping, high precision dating of zircons, and AMS analysis suggest that the ca. 500m thick external zone is the oldest intrusion batch, ca. 500Ka older than the internal part. It has a strong vertical lineation, and steep contacts. Since static thermal models do not provide for enough heat, we interpret the outer most 500m to be a feeder zone. Isogrades in contact metamorphic pelites were mapped and provide robust temperature estimates. These include - as a function of distance from the intrusive contact: andalusite (525°C, 1700m) and partial melting (675°C, 350m). Phase diagram calculations for 22 host-rock compositions reveal these estimates to be robust (e.g. not dependent on the chemistry of the host rocks). The data and thermal models suggest a lifetime of the feeder zone of a few thousand years.