The Suursaari conglomerate (SE Fennoscandian shield; Russia)-Indication of cratonic conditions and rapid reworking of quartz arenitic cover at the outset of the emplacement of the rapakivi granites at ca. 1.65 Ga

作者：Pokki Jussi^*; Kohonen Jarmo; Ramo O Tapani; Andersen Tom

来源：Precambrian Research, 2013, 233: 132-143.

DOI：10.1016/j.precamres.2013.04.008

• Detrital zircon
• Conglomerate
• Rapakivi granite
• Mid-Proterozoic
• Russia
• Fennoscandian shield

Ultramature, quartz arenitic conglomerate of the Suursaari Island, Gulf of Finland, Russia is one of the oldest known post-Svecofennian sedimentary rocks on the Fennoscandian shield and provides an important sedimentary record from the period between the collapse and resultant stabilization of the Svecofennian orogen (at ca. 1.8 Ga) and the emplacement of the first anorogenic rapakivi granites (at ca. 1.65 Ga). The Suursaari conglomerate is considered a remnant from a previously unknown depositional stage involving formation of an ultramature mid-Proterozoic sedimentary cover in purely cratonic conditions. U-Pb detrital zircon ages from the conglomerate peak at 1.88 Ga and 1.90 Ga implying Svecofennian synorogenic granitoids as the dominant source. Svecofennian metasedimentary rocks are the most likely source for the observed Paleoproterozoic (2.05-1.90 Ga) and Archean (3.3-2.5 Ga) zircon ages. Moreover, ca. 7% (ten grains) of the zircons recovered from the quartz arenitic matrix of the Suursaari conglomerate are of rapakivi age (ca. 1.65 Ga). These constrain a maximum depositional age for the conglomerate and imply that the Svecofennian orogen had reached continental peneplane prior to the rapakivi stage. The Suursaari conglomerate is thus one of the earliest marks of the initiation of a major intracratonic rifting stage. Rapid recycling of lithified sediment resulting in the deposition of the conglomerate was probably the result of accelerated erosion caused by block movements at the rapakivi stage (incipient, aborted rifting). The current exposure of the rapakivi plutons suggests rejuvenation of the topography of the peneplane at ca. 1.65 Ga and significant localized erosion of the Svecofennian roof of the rapakivi plutons. This eventually led to enhanced erosion that tapped lithologic units beneath the ultramature sedimentary cover and to the deposition of the immature Mesoproterozoic %26apos;Jotnian%26apos; arkosic redbeds, preserved in fault-bounded blocks in the vicinity of the rapakivi granite intrusions. The Suursaari conglomerate is compositionally comparable to the quartzites of the extensive cratonic cover sequences of midcontinental North America but it was not deposited in a subsiding, transient extensional basin setting.

• 出版日期2013-8