Paper No. 12
Presentation Time: 8:00 AM-12:00 PM
A 1:6000 MAP OF AZTEC WASH PLUTON, ELDORADO MOUNTAINS, NEVADA: DOCUMENTING THE ARCHITECTURE OF A STRATIFIED, MULTIPLY RECHARGED INTRUSION
Aztec Wash pluton is broadly divisible into two zones, one entirely granite (Granite Zone, GZ), the other larger and exceptionally heterogeneous (42-78% SiO2; Heterogeneous Zone, HZ). Steep tilting associated with extension exposes thick sections through the pluton. Previous work has noted extreme diversity of lithologies, evidence for interaction between mafic and felsic magmas, and stratigraphic arrangement of units. Full understanding of the architecture and construction of the pluton has been precluded by scale of diversity: important units exposed on scales of < 10's of m necessitate very detailed mapping. A new 1:6000 map clarifies the physical features of the 52 km2 pluton and confirms its stratigraphic assembly. Widespread indicators of original orientation, including felsic pipes and flame structures that penetrated overlying mafic sheets, load casts at the bases of sheets, and asymmetries within mafic-felsic layered sequences document the pattern of tilting and permit reconstruction of a ~6 km thick depositional sequence. Lateral interfingering of the GZ and HZ indicate synchronous deposition of GZ cumulate granites and HZ mafic flows. HZ stratigraphy comprises hundreds of repetitions of the sequence: (1) felsic cumulate (~1 cm-20 m); (2) mafic sheet (~1 to 50 m) consisting of (a) quenched base; (b) ±gabbroic interior with mafic pegmatite pods; (c) grading to less mafic upper zone including (d) variable thickness of fine-grained hybrid. Individual sequences are laterally continuous for up to >100 m and related sets of sequences for several km. Individual sequences and sets of sequences are locally disrupted by breccia zones that mark collapse and/or replenishment. Upper contacts between HZ tongues and overlying GZ are marked by thick accumulations (tens of m) of fine-grained hybrids and clasts of hybrid material in cumulate granite matrix. Country rock blocks (m->100m) are stratigraphically localized, with the largest accumulations in and above the upper part of the HZ. Mapping reinforces interpretations that the Aztec Wash chamber behaved as a complex depositional system during pluton construction. Products of repeated mafic replenishments interacting with resident felsic and hybrid magmas are manifested in system-wide stratigraphy and evidence for gravitational instability and mass flow.