The prevailing view of the formation of porphyry copper deposits along convergent plate boundaries involves deep crustal differentiation of metal-bearing juvenile magmas derived from the mantle wedge above a subduction zone. However, many major porphyry districts formed during periods of flat-slab subduction when the mantle wedge would have been reduced or absent, leaving the source of the ore-forming magmas unclear. Here we use geochronology and thermobarometry to investigate deep crustal processes during the genesis of the Late Cretaceous–Palaeocene Laramide Porphyry Province in Arizona, which formed during flat-slab subduction of the Farallon Plate beneath North America. We show that the isotopic signatures of Laramide granitic rocks are consistent with a Proterozoic crustal source that was potentially pre-enriched in copper. This source underwent water-fluxed melting between 73 and 60 Ma, coincident with the peak of granitic magmatism (78–50 Ma), porphyry genesis (73–56 Ma) and flat-slab subduction (70–40 Ma). To explain the formation of the Laramide Porphyry Province, we propose that volatiles derived from the leading edge of the Farallon flat slab promoted melting of both mafic and felsic pre-enriched lower crust, without requiring extensive magmatic or metallogenic input from the mantle wedge. Other convergent plate boundaries with flat-slab regimes may undergo a similar mechanism of volatile-mediated lower-crustal melting.

关于沿会聚板块边界形成斑岩铜矿床的普遍观点涉及来自俯冲带上方地幔楔的含金属幼年岩浆的深层地壳分化。然而，许多主要斑岩区是在平板俯冲期间形成的，此时地幔楔形可能已经减少或不存在，因此成矿岩浆的来源尚不清楚。在这里，我们使用地质年代学和温气压学来研究亚利桑那州晚白垩世-古新世拉拉米德斑岩省成因期间的深部地壳过程，该省是在北美下方法拉隆板块的平板俯冲期间形成的。我们表明，Laramide 花岗岩的同位素特征与可能预先富集铜的元古代地壳来源一致。这个源头在 73 到 60 马 之间经历了水熔融，与花岗岩浆作用的峰值 （78-50 马）、斑岩成因 （73-56 马） 和平板俯冲 （70-40 马） 同时发生。为了解释拉拉米德斑岩省的形成，我们提出，来自 Farallon 平板前缘的挥发物促进了镁铁质和长英质预富集下地壳的熔化，而不需要来自地幔楔形的大量岩浆或成矿输入。其他具有平板状态的会聚板块边界可能会经历类似的挥发性介导的下地壳熔化机制。