New Mexico Geological Society Annual Spring Meeting — Abstracts

A fracture generation model proposed by Dutrow and Norton (1995, JMG 13) predicts that the heating resulting from pluton intrusion into fluid-saturated, low permeability host rock will repeatedly increase pore fluid pressure and cause multiple fracturing events at individual localities. At conditions of 3 kbar and 800°C, the model predicts the greatest fracture density approximately 100 m from the pluton contact. I used quartz veins as proxies for fracture propagation in a natural contact aureole in order to test the assumptions and predictions made by this model.

The contact aureole ofthe 1.4 Ga Sandia Pluton records peak conditions of around 3 kbar and 750°C. Field observations indicate that quartz veins are most abundant 400-1000 m from the pluton contact and are rare close to the contact. XRF analyses indicate a 6 wt. % decrease in SiO₂ and an increase in other elements in the host schist immediately adjacent the vein in one sample location, whereas no depletion selvage has been determined in other sample locations. δ¹⁸O values of quartz at ~400 m from the contact are 13.3 per mil in the vein and 13.6 per mil in the host rock.

The spatial distribution of quartz in veins, SiO₂ depletion in vein selvages, and host-rock buffered δ¹⁸O values all support the model prediction that sequential heating of the country rock induced multiple fracturing events via in situ increase in pore fluid pressure. However, the absence of depletion selvages at 3-4 m wide veins suggests far traveled, plutonic fluids. I prefer a model in which initial heating increased the in situ pore fluid pressure initiating fracturing, and produced quartz veins and a new aureole fluid network system; subsequently, pluton derived fluids would have a conduit to migrate outwards through the aureole. Fluids in the aureole were thus both internally and externally derived, and fracturing events likely resulted initially from buildup of in situ pore fluid pressure followed by an increase in magmatic fluid pressure.