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

Mélanges represent a significant component of collisional and accretionary orogenic belts. We present a comparison of subduction-accretion mélanges of different ages to test how mélange-forming processes at convergent margins changed through time. We describe the Mesozoic McHugh mélange in Alaska, and the Paleozoic Proto-Tethyan Laohushan mélange of the Qilian Orogenic Belt of the Alpine-Himalayan chain, together with documentation of a Neoarchean mélange that marks the suture between the Eastern Block and Central Orogenic Belt of the North China Craton (NCC). The McHugh is a classic accretionary mélange that has not yet experienced a collision, and the Laohushan is an accretionary/collisional mélange with structures related to both stages of its evolution. We compare the characteristics of these mélanges with the world's largest Archean mélange, the newly-documented circa 2.5 Ga 1600 km long Jianping-Zunhua-Zanhuang-Dengfeng (JZZD) mélange of the North China Craton. The JZZD is a classical mélange 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 mélange 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 mélange 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 mélange contains relatively intact 2.5 Ga ophiolitic sheets in a 2.5 Ga metasedimentary mélange, intruded by 2.48 Ga granitic dikes. The Zunhua ultra-high pressure (UHP) podiform chromite mélange 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 mélange 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 mélange includes upper-plate blocks of metagabbro, metabasalt, BIF, and felsic rocks in a sandy to pelitic matrix, thrust over a turbiditic broken-formation mélange 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 (<650 °C/GPa) with tectonic burial in the accretionary wedge to at least 30 km, making this the world's first well-documented Archean paired metamorphic belt. This belt of mélanges records the life-cycle of an Archean subduction zone, from subduction initiation, processes of fore-arc oceanic crust and mantle generation during arc evolution, their incorporation as the upper plate in an accretionary mélange, and later disruption during collision, emplacement, and imbrication with the older passive margin of the Eastern Block on the lower plate, marking the death of the subduction zone. Arc polarity reversal 5–10 Ma after collision records the birth of a new subduction zone outboard of the accreted arc, and continued accretion of outboard terranes to the proto-North China craton. The mélange was metamorphosed at greenschist to amphibolite grade at 2.5 Ga, and locally up to granulite facies at 1.85 Ga during amalgamation of the NCC with the Nuna Supercontinent. Structures, rock types, subduction zone geothermal gradients (low-intermediate dT/dP), paired with high dT/dP in the arc section, and the history of the forearc mélanges of the Central Orogenic Belt are remarkably similar to those of the younger mélanges, except for the absence of blueschists and biogenic cherts, showing that convergent margin processes have remained similar for the past 2.5 billion years, and that plate tectonics was extant on Earth at least by the end of the Archean. The combination of rock types, structures, ages, geochemistry, and metamorphic relationships show that the Central Orogenic Belt is the world's first documented Archean paired metamorphic UHP orogen.