Structures, strain analyses, and ⁴⁰Ar/³⁹Ar ages of blueschist‐bearing Heilongjiang Complex (NE China): Implications for the Mesozoic tectonic evolution of NE China

Abstract

The high‐pressure Heilongjiang Complex belongs to a Jurassic accretionary complex in Yilan area, NE China. The complex is located in the junction of three orogenic systems in the NE Asia: the E‐W‐striking Central Asian Orogenic Belt (CAOB), the N‐S‐striking Paleo‐Pacific system, and the ENE‐WSW‐striking Mongol‐Okhotsk system. Thus, a detailed tectono‐metamorphic analysis coupled with accurate radiometric dating of the Heilongjiang Complex provided in this study represent important evidence for understanding the complex interactions between the three orogenic systems in the Late‐Palaeozoic to Mesozoic. New structural and microstructural data combined with the strain analysis highlight the presence of three distinct deformation events affecting the high‐pressure Heilongjiang Complex. The first deformation event D1 is related to development of blueschist facies prolate to plane strain fabrics that were subsequently reworked by the blueschist‐amphibolite facies event D2 that transposed all previous structures by the main west‐dipping schistosity. This planar fabric was folded by upright F3 folds and heterogeneously reworked by greenschist facies E‐W‐striking vertical cleavage during large scale N‐S shortening. Each deformation event was related to specific strain fabrics and metamorphic conditions highlighting dynamic evolution of the subduction complex during burial, exhumation, and final shortening. The first D1 event is related to westward directed prograde constrictional flow and progressive dismembering of competent mafic, ultramafic layers, and quartzites in highly stretched meta‐sedimentary matrix. This mélange was folded by recumbent detachment folds resulting in emplacement of the exhumed unit progressively reaching amphibolite facies conditions atop of W‐WSW‐dipping channel. This event is associated with dominant flattening implying increasing mechanical coupling between the colliding Songliao and Jiamusi blocks. Subsequently, the accreted subduction complex was reworked during N‐S‐directed horizontal shortening. This event was associated with an important post‐buckle flattening of fold limbs and rotation of competent blocks with inherited D1 and D2 strain ellipsoids along F3 fold axes. Our new 40Ar‐39Ar plateau ages on muscovite and phengite (173–177 Ma), together with previous radiometric data, are interpreted as reflecting exhumation of subduction channel during the blueschist‐amphibolite facies event while the N‐S shortening (D3) probably operated in Late Jurassic as indicated by some previously published cooling ages (~140–160 Ma). The westward‐oriented subduction and exhumation fabrics of the high‐pressure Heilongjiang accretionary complex followed by N‐S‐directed upright folding are interpreted to reflect transition from E‐W Pacific subduction regime to N‐S‐directed shortening related to the closure of the Mongol‐Okhotsk Ocean.

Convergence History of the Songliao and Jiamusi Blocks in the Eastern End of Central Asian Orogenic Belt: Evidence from Detrital Zircons of Late Paleozoic Sedimentary Rocks

“…The youngest metamorphic rock which protoliths are OIB igneous rocks yielded the age of ~140 Ma and ~180 Ma (Zhu et al, 2017a, b). Syntectonic mica Ar-Ar ages are distributed in ~140 Ma and ~160-180 Ma (Wu et al, 2007;Li et al, 2009Li et al, , 2011Aouizerat et al, 2019), these evidences limit the closure time of the Mudanjiang Ocean in the Middle Jurassic to Early Cretaceous. Ge et al (2017) reported that the oldest MORB-type rock was the Late Permian and OIB-type rocks was the Early Permian.…”

“…Because the blue schist in this unit is a significant sign of orogenesis, it has clockwise P-T-t paths (Bai et al, 1988;Zhang, 1992;Zhou et al, 2009;Zhao et al, 2010;Li et al, 2010Li et al, , 2014, researchers gradually realized that Heilongjiang complex is a tectonic melange, which is the product of the combination of Songliao and Jiamusi blocks (Cao et al, 1992;Zhang, 1992;Ye et al, 1994). There are many viewpoints on the assemblage time reflected by the Heilongjiang Complex, including Proterozoic (Cao et al, 1992), Early Paleozoic (Xie et al, 2008) and Late Paleozoic (Li et al, 2009), but a lot of recent chronological evidence indicates that the final assemblage time of the two blocks should be Middle Jurassic to Early Cretaceous (Wu et al, 2007;Zhou et al, 2009;Li et al, 2009Li et al, , 2011Zhu et al, 2017a, b, c;Aouizerat et al, 2019). Early Paleozoic syn-and post-orogenic granitoids Wang et al, 2012;Wang et al, , 2017 along both sides of the Mudanjiang suture zone and Devonian strata (Meng et al, 2010;Li et al, 2019) with similar age composition of detrital zircons show the possibility that Songliao and Jiamusi blocks were once integrated during the Paleozoic.…”

Heilogjiang Complex – Fragment of the Jurassic Accretionary Wedge in the Tectonic Windows of the Overlying Plate: Flat-Slab Subduction Model

“…Юрские метаморфизованные терригенные породы (а местами -серпентиниты) составляли, по-видимому, матрикс меланжа, а карбон-пермские и триасовые метабазальты, как и ассоциирующие с ними метаморфизованные кремни и карбонатные породы, образовали глыбы и пластины в этом меланже. Прямые подтверждения таких взаимоотношений наблюдались в карьерах около г. Илан, где слюдистые сланцы содержат глыбы и пластины голубых сланцев, амфиболитов и кварцитов размером от долей метра до 200 м [21]. Кроме того, в Хейлунцзянском комплексе присутствуют монцогранитовые жилы адакитового типа с возрастом 175-161 млн лет [22].…”

“…Метаморфиты комплекса Хейлунцзян обнаруживают следы интенсивнейших многоэтапных деформаций. Как показали исследования [21,72,77], сланцеватость метаморфитов, отвечающая заключительному этапу деформаций, имеет стабильное ВСВ (60-90°) простирание, то есть ориентирована практически ортогонально по отношению к меридиональному Муданьцзянскому разлому. Углы падения сланцеватостиот относительно пологих (20-45°) до вертикальных; не исключены и опрокинутые залегания.…”

Preface: Tectonics of China

“…Together with the geochronological data, it is suggested that the final subduction/collision between the Jiamusi and Songliao blocks took place in the Jurassic, triggered by the Palaeo‐Pacific subduction system. Meanwhile, Aouizerat et al () carried out a detailed structural analysis and 40 Ar– 39 Ar dating of the blueschist for the Heilongjiang Complex which suffered three phases of deformation with 40 Ar– 39 Ar plateau ages on muscovite and phengite of 173–177 Ma interpreted as a cooling age. Finally, they suggested that the westward‐oriented subduction and exhumation fabrics of the high‐pressure Heilongjiang accretionary complex, followed by N–S‐directed upright folding, are interpreted to reflect transition from E–W Pacific subduction regime to N–S‐directed shortening related to the closure of the Mongol–Okhotsk Ocean in the north‐west.…”