Mélanges through time: Life cycle of the world's largest Archean mélange compared with Mesozoic and Paleozoic subduction-accretion-collision mélanges

Abstract Melanges represent a significant component of collisional and accretionary orogenic belts. We present a comparison of subduction-accretion melanges of different ages to test how melange-forming processes at convergent margins changed through time. We describe the Mesozoic McHugh melange in Alaska, and the Paleozoic Proto-Tethyan Laohushan melange of the Qilian Orogenic Belt of the Alpine-Himalayan chain, together with documentation of a Neoarchean melange that marks the suture between the Eastern Block and Central Orogenic Belt of the North China Craton (NCC). The McHugh is a classic accretionary melange that has not yet experienced a collision, and the Laohushan is an accretionary/collisional melange with structures related to both stages of its evolution. We compare the characteristics of these melanges with the world's largest Archean melange, the newly-documented circa 2.5 Ga 1600 km long Jianping-Zunhua-Zanhuang-Dengfeng (JZZD) melange of the North China Craton. The JZZD is a classical melange in terms of map patterns, lithological relations, structural characteristics, including mixed rock types of different ages, origins, and metamorphic histories, and preserves both paired metamorphic features and local blocks preserving ultra-high pressure (UHP) assemblages. It shows high stratal disruption, with both block-in-matrix, and highly-mixed thrust-bounded tectonic slices types of map patterns. The melange is part of an arc/accretionary complex that was emplaced over a 2.7–2.5 Ga passive margin sequence on the western edge of the Eastern Block of the NCC at 2.5 Ga. The frontal zone of the accretionary complex marks the suture between the Eastern Block and an oceanic arc terrane in the Central Orogenic Belt (COB), and exhibits significant along-strike N-S variations. In the north the Jianping pyroxenite melange contains blocks of 2.52 Ga metapyroxenite, metabasalt, tonalite-trondhjemite-granodiorite-diorite (TTGD) gneisses, in a metasedimentary and serpentinite matrix, intruded by 2.49–2.47 Ga plutons, metamorphosed to amphibolite-granulite conditions at 2.47 Ga. The Zunhua/Shangying ophiolitic melange contains relatively intact 2.5 Ga ophiolitic sheets in a 2.5 Ga metasedimentary melange, intruded by 2.48 Ga granitic dikes. The Zunhua ultra-high pressure (UHP) podiform chromite melange has blocks of pillow basalt, BIF, dikes, gabbros, pyroxenite, harzburgite, some with 2.55 Ga podiform chromites in dunite envelopes, enclosed in a 2.52–2.46 Ga metasedimentary matrix, metamorphosed and intruded by granitic dikes at 2.46 Ga. The chromites and dunite pods formed through reaction of a boninitic melt and harzburgite during subduction initiation bewtween 2.6–2.55 Ga. UHP conditions are documented by TiO2 II inclusions in chromites, indicating subduction to at least 270 km depth. The Zanhuang melange includes blocks of metapsammites, metagabbros, metabasalts, and ultramafic rocks in a metapelitic matrix, intruded by 2.49–2.54 Ga granites, then imbricated with >2.65 Ga marble-metapelite-quartzite of the passive margin of the Eastern Block. The Dengfeng melange includes upper-plate blocks of metagabbro, metabasalt, BIF, and felsic rocks in a sandy to pelitic matrix, thrust over a turbiditic broken-formation melange unit in the east. Circa 2.5 Ga metamorphic conditions record a relatively high T/P anatexis (>950 °C/GPa) in the west, whereas the central part of the belt records intermediate T/P metamorphism (