STRUCTURAL EVIDENCE FOR NESTED DIAPIRS IN THE FANGSHAN PLUTON, CHINA

The debate over pluton growth has lead to 3 end-member models for normally zoned pluton growth: formation by 1) fractionation in a single closed chamber; 2) small dikes or sills; 3) nested diapir-like pulses. The normally zoned Fangshan pluton located near Beijing, China is ideal for testing these models due to its good exposure and numerous prior studies. We evaluate the above models using structural evidence from the pluton.

The 8.5 km diameter Fangshan pluton contains 2 circular, concentric units: an outer quartz diorite forming a discontinuous zone partially intruded by the internally zoned, inner granodiorite. Complex mingling zones, xenoliths, and dikes of younger units define the internal contact. Magmatic foliations and lineations are steep, form a typical onionskin pattern, and overprint internal contacts. Weak constrictional fabrics with 2 ~orthogonal foliations and a vertical lineation occur at the pluton center, yielding to 1 more intense foliation and weak lineation near the margins. Mafic enclaves throughout the pluton are non-spherical and statistically aligned with the single outer and both inner magmatic foliations, but exceptions exist. On the north and west margins, strong high-temperature solid-state fabrics overprint the magmatic fabric, are cut by late dikes, and coincide with further enclave strain.

Contact relationships imply intrusion of 2 large, concentric, nested, magma pulses, the second pulse removing much of the first. We interpret the dominant lineation/weak foliations to show constrictional strain in the center, while the strong foliations show flattening strain near the margin, consistent with flow in a large chamber rather than in small dikes or sills, and contradictory to simple ballooning models involving pure flattening strain. Steep lineations challenge a previous model of westward lateral pluton growth based on shallow lineations and the western solid-state shear zone. We instead support vertically rising, nested diapirs.