Yes, Haplogroup I1
itself appears to be Chalcolithic in age, even though Haplogroup I
as a whole is probably Mesolithic in age (we know that I2a was present in the Neolithic in the Atlantic region, thereby making Haplogoup I a probably candidate for a Mesolithic European haplogroup). From this perspective, it would seem most plausible that I1 is essentially a single surviving male lineage that coincidentially survived inside a population of otherwise Chalcolithic newcomers. The case for this discontinuity can also be made
elsewhere.
The question is, can the spread of Haplogroup I be solely explained by the migrations period and by the spread of the Germanic peoples? Even though without a doubt the Haplogroup was spread by the Germanic peoples, I'm not quite convinced that the distribution pattern we see can be solely explained by this. In particular, there is a significant percentage amongst the Finnic peoples which in my opinion can not be explained solely through interaction with the Vikings (we are talking about some of the highest frequencies of the Haplogroup here, after all). Another issue is the surprisingly high concentrations in the Atlantic region, especially in Ireland, Wales and on the Iberian pennsula. While it would be possible to attribute this to the Vikings (in Ireland) and the Suebi (in Galicia), I would ask the question if it is possible that there a
small Celtic component to Haplogroup I1 as well. Specifically, if Haplogroup I1 was already present in Central Europe by the start of the iron age, and subsequently spread with the Hallstatt and La-Tene Cultures, this would better explain some of the distribution patterns we see in the Atlantic region (as well as in some other areas, such as along the Danube), rather than if we solely assume a spread by the Germanic peoples during the Migration Period.
The reason I ask this is because the distribution in Iberia corresponds better with the maximum extend of Celtic influence on the Iberian penninsula (3rd century BC) than it does with Germanic influence (the Suebi are often cited as the source of I1 in Iberia). However, what about the Visigoths, why were they not such significant bearers of I1 if the Suebi were?