## 4 Discussion
### Genesis of Amphibole Crystal Population
#### 4.1.3 The Crystallization Process of Amphibole Crystal Population  
As noted earlier, the Type 1 amphibole is an autocryst with Mg# ranging from 65 to 68, formed directly from the host magma after intrusion into the shallow crust. The Type 2 idiomorphic amphibole, with typical cleavages, is characterized by negative Eu and Sr anomalies (Figure 7), indicating prior plagioclase crystallization differentiation in the parent magma. It formed in a shallower reservoir (9.5-19.7 km) with lower P and T and higher fO\({}_{2}\) (Figure 10) compared to the Type 3 and Type 4 amphiboles. This euhedral crystal is nearly homogeneous, with only narrow chemical zoning at the rim. The homogeneous interior formed during the early stage may record the initial stable crystallization conditions (Figure 5). The narrow zoning at the rim exhibits low REE contents and elevated Th and U contents, indicating overgrowth after capture by the host magma. The Type 3 amphibole is also euhedral, with typical cleavages. As the upper temperature limit for amphiboles is typically around 1050 \({}^{\circ}\)C [12], the average crystallization temperature of  
Figure 10: Diagrams for selected amphiboles determined by the amphibole thermobaremeter [55,60]: (**a**) P–T; (**b**) logfO\({}_{2}\)–T; (**c**) T–H\({}_{2}\)O\({}_{\rm melt}\). Bt = biotite, PI = plagioclase, Opx = orthopyroxene, Cpx = clinopyroxene, Mgn = magnetite, llm = ilmenite, Ol = olivine, Mg-Hbl = magnesiohornblende, Tsc-Prg = tschemrakitic pargasite. 1010-1030 \({}^{\circ}\)C suggests that this amphibole is in a liquidus phase, formed during the cooling of the parent melt. Before the amphibole formation, most of the olivine and clinopyroxene crystallized at higher temperatures and separated from the residual melt via fractional crystallization, with some retaining as free crystals. This process provided more space for amphibole crystallization. This may explain why amphiboles in the Sanggeda gabbros are mostly euhedral, unlike the interstitial forms found in other gabbros worldwide [19; 23; 61]. This is further supported by the coexistence of clinopyroxene and the Type 4 amphibole. The high Cr content (1199-1600 ppm, Table S4), Mg# (80.87-83.69, Table S2), and low trace element concentrations (Figure 7) of clinopyroxene suggest formation from a primitive parent melt. This also accounts for the high water content (4.9-7.4 wt.%) in the parental melt of the amphiboles. The crystallization of anhydrous minerals (olivine and clinopyroxene) causes relative enrichment of water in the melt, as mafic arc magmas typically contain around 4 wt.% water [62; 63]. High water content in the basaltic melt suppresses the crystallization of plagioclase [63; 64; 65] and promotes the stability of clinopyroxene compared to orthopyroxene [66; 67]. This aligns with the observation that plagioclase and orthopyroxene are rare in the cumulate and that amphiboles or clinopyroxene shows no significant negative anomaly of Eu. The Mg# in the Type 3C amphibole decreases gradually, indicating that the magma system stabilizes during crystallization without melt recharging. The high compositional consistency between the rim and core Type 3C amphibole may be due to limited influence from the host magma, as it is surrounded by cumulates. The Type 4 amphibole appears as a clinopyroxene pseudomorph (Figure 3b,c), indicating a close relationship with clinopyroxene. Amphiboles formed by the reaction-replacement of clinopyroxene typically retain remnants of precursor clinopyroxene [19; 23; 24; 61]. However, no relics of clinopyroxene are observed in the Type 4 amphibole. This may result from extensive and prolonged reactions between the precursor clinopyroxene and water-rich magma [19; 68; 69], consistent with the high water fugacity (5.1-7.4 wt.%). Additionally, most clinopyroxene, which separates from the magma via fractional crystallization at an early stage, may promote this reaction.