Abstract The Machangqing porphyry Cu-Au deposit is located in the Sangjiang region, Jinshajiang-Ailaoshan metallogenic belt, southeastern Tibet. It has three main phases of felsic-mafic intrusions: barren granites, ore-forming porphyry intrusions, and mafic lamprophyres. U-Pb zircon dating shows that these intrusions were emplaced over a period of ~3 m.y., with lamprophyres at 36.50 ± 1.6 Ma (1 σ ), porphyry intrusion at 34.26 ± 0.22 Ma (1 σ ), and granite intruded at 34.00 ± 0.26 Ma (1 σ ). The in situ Rb-Sr analysis of phlogopite and amphibole, primary minerals in the lamprophyres, also gives a date of 36.5 ± 1.5 Ma (2 σ ), regarded as the emplacement age of the lamprophyre, earlier than the ore-forming porphyry intrusions. The magmatic phases have significantly different sulfur and chlorine contents. The SO 3 contents of igneous apatite microphenocrysts from the mineralization-related porphyry intrusions are higher (0.24 ± 0.14 wt %, 1 σ , n = 82) than those from the barren granites (0.08 ± 0.07 wt %, 1 σ , n = 30). The chlorine contents in apatite grains from the porphyry intrusions (0.18 ± 0.16 wt %, 1 σ ) are also higher than those from granites (0.04 ± 0.02 wt % Cl, 1 σ ). The apatite in lamprophyres have higher sulfur (0.68 ± 0.19 wt %, 1 σ , n = 40) and chlorine (0.48 ± 0.13 wt %, 1 σ ). The large difference of Cl and S in lamprophyres might suggest that elevated magmatic volatile contents derived from the mafic magma were important for ore formation in the Machangqing porphyry systems. The in situ Sr and O isotopes in apatite phenocrysts from the porphyry intrusions ( 87 Sr/ 86 Sr: 0.70593–0.70850; δ 18 O: 6.0–7.0) are similar to those in the lamprophyres ( 87 Sr/ 86 Sr: 0.70595–0.70964; δ 18 O: 5.4–6.9), consistent with similar origins for their volatile contents. These data may indicate that the deeper magma chamber was recharged by a relatively S-Cl-rich mafic magma similar to the lamprophyres, triggering the ore-forming magmatic event. This study also suggests that origin of apatite Sr and also volatile contents, combined with in situ Sr and O isotopes, could be useful for fingerprinting fertile intrusions associated with mineralization within drainage source areas or in outcrops.

ABSTRACT Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al 2 O 3 (5.53–13.63 wt%) and crystallized at ~1300–1500 °C and 16–30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950–1100 °C and 12–18 kbar before entrainment in the basanites. Rare earth element (REE) thermobarometry of garnets and coexisting clinopyroxenes preserves evidence of an intermediate stage (1032 °C and 21 kbar). These results imply that the protoliths of the garnet pyroxenite formed at a range of depths from ~50 to 100 km, and then during or shortly after cooling, they were tectonically emplaced to higher levels (~40–60 km; i.e., uplifted by at least 10–20 km) along the prevailing geotherm. This uplift may have been connected with lithosphere-scale faulting during the Paleozoic orogeny, or during Mesozoic–Cenozoic rifting of eastern Australia.