The Acoje block of the Zambales ophiolite in the Northwest Philippines is a remnant of forearc lithosphere that formed during an Eocene subduction initiation event in the Western Pacific Ocean. It hosts abundant high-Cr chromitites in the uppermost part of the mantle sequence and the overlying dunitic Moho transition zone. The host harzburgites of the uppermost mantle sequence vary upward from Cpx-rich to Cpx-poor varieties. Clinopyroxene grains in both types of harzburgites and the dunites have prevalent finger-like protrusions and diopsidic compositions, indicative of melt impregnation. From the Cpx-rich harzburgites to Cpx-poor harzburgites and dunites, the clinopyroxene grains display increasingly depleted major element and rare earth element (REE) compositions, e.g., Al₂O₃ contents from 5.15 wt% to 0.84 wt% and Yb concentrations from ~ 0.2 ppm to 1.5 ppm. These changes are compatible with a transition from MORB to boninitic affinity, Olivine grains in the dunites and sparsely disseminated chromitites vary widely in δ⁷Li values, primarily from ~ +1 to +10. Such a δ⁷Li range implies that the parental magmas of the high-Cr chromitites were derived from mantle sources that were modified by slab fluids and had Li isotopic signatures either similar to, or heavier than, the normal mantle range (+4 ± 2). Partial melting in the mantle sources was triggered by heating of previously hydrated Cpx-rich harzburgites during slab rollback and subsequent asthenospheric upwelling. The upwelling brought asthenospheric materials and high heat flux into the lithospheric mantle, facilitating formation of high-Ca boninitic magmas and high-Cr chromitites in the nascent mantle wedge. The different Li isotopic domains in the mantle sources of the high-Cr chromitites indicate that the mantle temperatures were elevated rapidly after the hydration event in order to generate the boninitic magmas.

菲律宾西北部的Zambales蛇绿岩的Acoje块体是在西太平洋始新世俯冲引发事件期间形成的前臂岩石圈的残余。它在地幔序列的最上部和上覆的双元莫霍过渡带中蕴藏着大量的高铬铬铁矿。地幔序列最上部的寄主哈氏石从富含Cpx的品种到缺乏Cpx的品种向上变化。两种类型的尖晶石和辉石岩中的次氯环己烯晶粒均具有普遍的手指状突起和二分相组成，表明熔体浸渍。从富含Cpx的哈氏石到贫Cpx的哈氏石和dunites，斜辉石晶粒显示出越来越少的主要元素和稀土元素（REE）成分，例如Al ₂ O ₃含量从5.15 wt％到0.84 wt％，Yb浓度从〜0.2 ppm到1.5 ppm。这些变化是与从MORB到boninitic亲和性的过渡兼容，在纯橄岩橄榄石颗粒和疏播散铬铁矿在δ广泛变化⁷李值，主要是从〜1到10。这种δ ⁷Li范围意味着高Cr铬铁矿的母岩浆来自由地幔流体修饰的地幔源，并具有与正常地幔范围相似或重于其的Li同位素特征（+4±2）。在板坯回滚和随后的软流圈上升过程中，加热先前水合的富含Cpx的哈茨石会触发地幔源的部分熔融。上升流将软流圈物质和高热通量带入岩石圈地幔中，促进了新生地幔楔中高钙贝氏体岩浆和高铬铬铁矿的形成。高Cr铬铁矿地幔源中不同的Li同位素域表明，水合事件发生后地幔温度迅速升高，从而产生了贝尼特质岩浆。