Editing K2 (section)

==Geology==
The mountains of K2 and [[Broad Peak]], and the area westward to the lower reaches of [[Sarpo Laggo glacier]], consist of [[metamorphic rock]]s, known as the ''K2 Gneiss'' and part of the Karakoram Metamorphic Complex.<ref name="Searle1991a">{{cite book|author=Searle, M.P. |date=1991|title=Geology and Tectonics of the Karakoram Mountains|publisher= John Wiley & Sons|location= New York City|page= 358 |isbn=978-0471927730}}</ref><ref name="Searle1991b">{{cite book|author=Searle, M.P. |date=1991|title=Geological Map of the Central Karakoram Mountains.'' scale 1: 250,000''|publisher= John Wiley & Sons|location=New York City}}</ref> The K2 Gneiss consists of a mixture of [[orthogneiss]] and [[biotite]]-rich [[paragneiss]]. On the south and southeast face of K2, the orthogneiss consists of a mixture of a strongly [[Foliation (geology)|foliated]] [[plagioclase]]-[[hornblende]] [[gneiss]] and a biotite-hornblende-[[K-feldspar]] orthogneiss, which has been intruded by [[garnet]]-[[mica]] [[Leucogranite|leucogranitic]] [[Dike (geology)|dikes]]. In places, the paragneisses include [[clinopyroxene]]-hornblende-bearing [[psammite]]s, [[garnet]] (grossular)-[[diopside]] [[marble]]s, and biotite-[[graphite]] [[phyllite]]s. Near the memorial to the climbers who have died on K2, above Base Camp on the south spur, thin impure marbles with quartzites and mica schists, called the ''Gilkey-Puchoz sequence'', are interbanded within the orthogneisses. On the west face of Broad Peak and the south spur of K2, [[lamprophyre]] dikes, which consist of clinopyroxene and biotite-[[porphyritic]] [[vogesite]]s and [[Lamprophyre|minette]]s, have intruded the K2 gneiss. The K2 Gneiss is separated from the surrounding [[sedimentary]] and metasedimentary rocks of the surrounding Karakoram Metamorphic Complex by [[Fault (geology)|normal faults]]. For example, a fault separates the K2 gneiss of the east face of K2 from [[limestone]]s and [[slate]]s comprising nearby [[Skyang Kangri]].<ref name="Searle1991a"/><ref name="SearleOthers1990a"/>

[[Argon–argon dating|<sup>40</sup>Ar/<sup>39</sup>Ar ages]] of 115 to 120 million years ago obtained from and geochemical analyses of the K2 Gneiss demonstrate that it is an older, metamorphosed, pre-collisional [[Cretaceous]] [[granite]]. The granitic precursor ([[protolith]]) to the K2 Gneiss originated as the result of the production of large bodies of [[magma]] by a northward-dipping [[subduction zone]] along what was the [[continental margin]] of Asia at that time and their intrusion as [[batholith]]s into its lower [[continental crust]]. During the initial collision of the Asia and Indian plates, this granitic batholith was buried to depths of about {{convert|20|km}} or more, highly metamorphosed, highly deformed, and partially remelted during the Eocene Period to form gneiss. Later, the K2 Gneiss was then intruded by leucogranite dikes and finally exhumed and uplifted along major breakback thrust faults during post-Miocene time. The K2 Gneiss was exposed as the entire K2-Broad Peak-Gasherbrum range experienced rapid uplift with which erosion rates have been unable to keep pace.<ref name="Searle1991a"/><ref name="SearleOthers2010a"/>