T ÉCNICAS FORMALES EMPLEADAS EN EL PROCESO DE DESARROLLO DE SOFTWARE LIBRE

Little is known about when the remaining open part of the “Luliang” Ocean (Trap et al., 2012) closed, since most workers in China have simply assumed that the borders of the TNCO as defined by Zhao (2001) represent the borders along which the eastern and western blocks

collided at 1.85 Ga. However, a few studies have looked deeper into this “second suture”, and found several features that indicate that the closure time was, may be, at 2.5-2.4, 2.3, 2.1, or 1.85 Ga.

Faure et al. (2007) and Trap et al. (2007, 2008, 2009a, b, 2012) provided detailed structural documentation of a suture that they termed the “Trans North China Suture” along the western side of the accreted Wutai/Fuping Arc terrane. This suture is best exposed in the Luliang

ACCEPTED MANUSCRIPT

Massif (LL on Fig. 17) where it crops out as greenschist facies metasedimentary rocks mixed with mafic and ultramafic rocks, which have an oceanic affinity (Trap et al., 2009b; 2011; Polat et al., 2005). Trap et al. (2011) included the flysch, mafic rocks, pillow basalts and other

sedimentary rocks of the Wutai Greenstone Belt in an allochthonous unit (called the Low Grade Mafic Unit, LGMU in their terminology) that was extruded from the Trans North China Suture and thrust over TTGs and migmatites of the Fuping Block (Fig. 7). Interestingly, the pillow basalts, gabbros, and felsic volcanic rocks in this suite have ages of 2.530-2.515 Ga (Wilde et al., 2005) whereas the underlying TTG gneisses of the Fuping Complex have ages of 2.560-2.515 Ga (Wilde et al., 2005). This in turn implies that the Luliang Ocean closed not too long after 2.5 Ga, since it is unusual for oceanic-affinity rocks in sutures to be older than the ocean formation age, and most are obducted soon after they form (Burke et al., 1977; Dewey, 1977). Strangely, Trap et al. (2012) interpreted these rocks to have formed in a basin that rifted from a previously

amalgamated NCC at 2.3-2.2 Ga, and therefore the Luliang Ocean closed at circa 1880 Ma.

It is clear that there were significant magmatic and partial melting events between 2.3 and 2.1 Ga in the Fuping, Wutai, and Hengshan areas, and also along the northern margin of the NCC, Ordos Block, Alxa Block, and Eastern Block (Fig. 17). In the Fuping/Wutai/Hengshan areas these events include evidence for anatectic melting from the Fuping Complex at 2193 +/- 15 Ma (Wang ZH et al., 2010), 2.06-2.08 Ga anatectic melts in the Fuping Complex (Cheng Y.Q. et al., 2001), and 2.25-2.11 Ga anatectic granites from the Hengshan Complex (Kröner et al. 2036 +/- 19 Ma 2008). Trap et al. (2012) interpreted these to reflect a regional anatectic melting event within the TNCO, and Zhao et al. (2008, etc.) interpreted them as products of Andean-type arc magmatism related to eastward subduction beneath the TNCO. In contrast, Kusky and Li (2003) interpreted these intrusions to form a wide zone of magmatism related to a convergent margin and Andean arc-related activities stemming from subduction under the northern margin of the

ACCEPTED MANUSCRIPT

craton, 200 km to the north, and stretching 1,600 km EW along the northern margin of the craton.

Note that magmatism associated with the present-day Andean arc extends some 500 km from the convergent boundary, so this is not an unusually large distance (Fig. 17). In recent papers Wan, Y.S., et al. (2013) and Zhang, C.L., et al. (2015) reported U-Pb ages and chemical data from circa 2.2-2.0 Ga granitic gneisses from boreholes in the Ordos basement, and interpreted them to be part of a continental margin arc. This is in perfect agreement with the interpretation that the northern margin of the craton was an Andean-style arc at this time (Kusky and Li, 2003, Kusky, 2011a), but it would have been impossible to produce, if the TNCO had not closed by then, and the Andean arc was located above an east-dipping subduction zone beneath the eastern NCC (e.g., Zhao et al., 2001, etc).

Very little happened between 2.4 and 2.3 Ga, suggesting that the Luliang Ocean closed very soon after the collision of the Fuping arc with the Eastern Block. This may be reflected in the ca. 2512±12 to 2469±6 Ma, 2458±12 to 2449±5 Ma, 2435±27 to 2385±7 Ma metamorphic ages obtained from the Jianping Complex (Liu, S.W., et al., 2011), the 2.44 Ga metamorphic ages from the Minyun Geopark (Shi, Y.R. et al., 2012), reflecting the initial collision of the

Wutai/Fuping arc with the Eastern Block, a subduction polarity reversal, then closure of the remaining open part of the “Luliang Ocean” some 70 Ma after the initial collision (Fig, 20). This agrees with data from the Zanhuang Massif, where granite plutons and pegmatites that cut the fabrics in the mélange have yielded ages of 2493 +/- 22 Ma, 2540 +/- 23 Ma, and 2539 +/- 44 Ma (Wang J.P. et al., 2013), which also cut mafic dikes that cut the mélange fabric with ages of 2535 +/- 30 Ma, and have metamorphic zircons with ages of 2.1 (with a large error) and 1.85 Ga (Deng, H. et al., 2014). We therefore suggest, based on this limited information, that the Luliang Ocean closed by 2435 Ma, and the Eastern and Western Blocks were amalgamated at this time (Fig. 20).

ACCEPTED MANUSCRIPT

The magmatic gap from 2435-2300 Ma represents the time between closure of the “Luliang”

Ocean, and when the Andean arc was set up along the northern margin of the craton.

The nature of the basement to the Western Block is enigmatic, since it is largely covered by late Archean to modern sedimentary rocks. However, Kusky and Mooney (2015) synthesized geophysical, geological, and geochronological data on the nature of the Ordos (part of the

Western Block) basement, and suggested that it is likely an oceanic plateau that was accreted (Fig.

20a, b), and experienced several periods of later differentiation during younger subduction and collision events along the northern margin of the craton.

Fig. 20 near here. Cross sections at 2.43 and 2.3 Ga