Types of Enclaves and Their Features and Origins in Intermediate-Acid Intrusive Rocks from the Tongling District, Anhui Province, China

Enclaves in intermediate-acid plutons from Tongling can be divided into three types: xenoliths, relics and magmatogenic enclaves. The magmatogenic enclaves consist of cumulates, micrograined dioritite mixtite and dioritic chilled border enclaves. Petrologically, relics with eyed and meta-poikilitic texture are characterized by high content of biotite (>80%) and low content of cordierite and grossular. The cumulates with accumulate texture consist of a great amount of pyroxene, hornblende and minor spinel and phlogopite. The micrograined dioritic mixtite is composed of more hornblendes and feldspar and less needle apatites and an ellipsoid basic core included in plagioclase. The chilled border enclaves have the same mineral association, but more dark minerals than the host rocks consisting of plagioclase, quartz, alkaline feldspar, hornblende and biotite. Geochemically, the relics exhibit high REE content (455.8×10−6) and high ratio of LREE/HREE, more obvious Eu negative anomaly and are rich in Cr and Ni, poor in Ba, Nb and Sr. All kinds of magmatogenic enclaves have the same REE pattern without Eu anomaly, and the total REE is 74.9×10−6 for spinel pyroxene cumulate, 179.7×10−6 for hornblende cumulate, 226.9 × 10−6 for hornblende cumulate crystal, 289.9×10−6 for micrograined dioritic mixtite and 131.2×10−6 for chilled border enclaves. Calculation with the thermometer and barometer for minerals shows that the temperature and pressure of formation are 1126 °C and 8.44 kbar (7.15–9.04 kbar) for the spinel pyroxene cumulate, 1065 °C-1029 °C and 8.23 kbar (7.13–9.50 kbar) for the hornblende pyroxene cumulate, 959 °C and 3.40 kbar (2.86–4.09 kbar) for the micrograined dioritic mixtite enclaves, and 951 °C and 0.59 kbar for the dioritic chilled border enclaves, respectively. Thus, the following conclusions can be drawn. Relic enclaves were derived from the crustal partial melting and all cumulate enclaves originated from deep magma chambers near the Moho in the lower crust, which are cumulate fragments formed by fractional crystallization of basic magma; mixtite came from shallow magma chambers in the middle-upper crust, formed by the mixture of mantle-derived magma and acid magma derived from the crustal partial melting; and the chilled border enclaves came from the border fragments of early intrusive bodies.